VENICE— A large international show like the 43d Venice Biennale is as much carnival and theater as it is an art exhibition. For as long as the art community is in attendance, which is usually for the three or four days just prior to the official opening, the show generates a kind of subculture, a world unto itself, with a rich cast of clowns, acrobats, ringmasters and strongmen.

The Biennale is the world's longest running art show. It began in 1895 and continues to grow. Its two dozen permanent national pavilions of varying sizes are in the Public Gardens that help provide this city of sickly water and glorious stone with a handful of trees. There are 40 countries represented in the pavilions and nearby former arsenal, the site of the Aperto, or Open exhibition of new art that is in fact not open but selected by an international committee.

If the Biennale can make your head spin, it is also fun. There is the sheer pleasure of being around a lot of the work. For the possibility of seeing new and established art in a context in which their strengths and weaknesses can be defined, the exhibition is invaluable.

What is discouraging, however, is the way large international exhibitions encourage people to blow through them. With all the national pavilions, and with an arsenal corridor so long that provisions should be offered along the way, the Biennale is enormous. When people meet after a couple of days, they may be as anxious to whine about their ankles and knees as they are to discuss art. In order to consider the show in an appropriate manner, one curator periodically returned to her hotel to soak her feet in ice water.

People move and judge fast. ''What did you like?'' they say between pavilions. ''Did you see anything great?''

What gets thrown back and forth is a multitude of impressions, observations and reactions. Listening to them is one of the rewards of the show. There is nothing wrong with the quick take, particularly when the person offering it is a professional.

After seeing the paintings of Jasper Johns in the United States Pavilion, one artist said, ''He lets me know how far I still have to go.'' One dealer remarked upon the supreme professionalism of Tony Cragg's sculpture installation in the British Pavilion -referring not just to the presentation of the art, but also to the way in which the labels on the wall managed diplomatically to acknowledge different dealers.

The problem is that size and speed go together. The art world is now filled with people for whom the quick take is all there is. There are all these people who cross the sea to visit this show, then race through it, then travel on, perhaps to the new museum of contemporary art in Prato, near Florence, or to the big sculpture show in West Berlin. When they arrive there, they will move just as fast. These are people with power in the art world. They include some of the best people we have.

What can someone get out of art when it is always seen from a moving train? What is it about art that makes them care? What does their relationship to art mean? Awash in New Yorkers

In the days before the June 26 opening of the Biennale, Venice seemed engulfed by American artists, curators, collectors, dealers and critics. ''All of New York is here,'' one collector announced with a mixture of pride and horror. Indeed, there were moments, not only at the Biennale itself, but also in the Piazza San Marco and in the streets and restaurants around the Fenice Theater, when he seemed to be right. The reason for the unusual American turnout was the respect for Jasper Johns, the 58-year-old painter who on the day of the opening was awarded the Biennale's top prize, as the best living artist in the show.

If there is going to be full-scale art-world interaction, it might as well be in Venice. The setting is not bad. The New Yorkers may be so happy to be there because Venice is another ''this is a great place to visit but I wouldn't want to live here'' city. Its beauty is not threatening to the New World because it seems so vulnerable amid the city's decay.

Venice also has a softness that is reflected in its cats. Even for an Italian city, the number of cats in Venice is staggering. Many are homeless, but women are always appearing out of nowhere to feed them. At least for one critic, watching bands of cats emerge at dusk, batter one another, stare down strangers and sleep dead on their sides in the center of public squares oblivious to everything but dogs is reassuring. In Venice, the art community can be warm, even nice. The Cast of Characters

Art professionals go to the Biennale for many reasons. Curators are usually there to inform themselves. Critics are there to cover the show.

With collectors, it is more complicated. They may be there to keep up, or because a work they own or an artist they support is in the show, or because they want to be part of what they perceive as the glamorous art scene, to which they believe collecting has bought them admission. If the work they own is important enough, they are right.

In order to have the privilege of paying a fortune for a painting by Jasper Johns, a collector has to be designated by the dealer as a significant person. Money alone will not buy a Johns. However, if you are fortunate enough to own one and generous enough to lend it to a museum for an exhibition, you will receive the sort of V.I.P. treatment usually reserved for museum directors or the artists themselves.

Of all the visitors to the Biennale, the most difficult to categorize are the dealers. Some are insiders in the best sense, and their informed and enthusiastic presence at these occasions is indispensable. Some dealers may be be there simply to take in the art and have a good time.

Almost all of them are always working, however, even when they do not appear to be. At the elaborate dinner thrown for Jasper Johns by the Philadelphia Museum of Art, the institution that organized his show, one New York dealer looked at the name cards at his table, decided no one there was important enough, and went elsewhere.

There is an indoor-outdoor cafe-restaurant at the entrance to the Biennale. Since there is no other cafe nearby, it is the stage for a great deal of activity. It is by the water, under trees, a good place to eat in if you have four hours. It is one of those places where the waiters seem to be working themselves to death, but no one is ever served. That the cafe-restaurant at the Biennale is surely the slowest in Venice says something about the stubborn incompetence exhibition officials have to deal with.

At this cafe-restaurant, many artists appear with their dealers, and the complexity of the artist-dealer relationship is readily apparent. When there is real friendship between artists and dealers, they are likely to enter quietly and take a place in a corner of their own.

But many of the relationships involve power. The way some artists show up with their dealers and plop themselves down at a particularly visible table announces to everyone that they have a dealer, and that the dealer is loyal and near.

There is also a strong need on the part of prominent dealers to be seen with their stars. There are dealers who walk into the cafe, size up its occupants and parade their artists like prize bulls. At the Venice Biennale, everyone gets gored, no one gets bored.

If you're in the neighborhood, the Biennale continues through Sept. 25.

Photo of Jasper Johns (Hans Namuth); the U.S. Pavilion at the Venice Biennale (NYT/Michael Brenson)