Yeast beta-(1 goes to 3)glucan synthetase is stimulated by ATP or GTP. The structural requirements for the activation were investigated by testing several phosphorylated compounds. The simplest substance with stimulatory ability was inorganic pyrophosphate. Addition of a nucleoside, as in GDP, decreased the concentration required for half-maximal stimulation; a third phosphate group, as in GTP, further enhanced the stimulatory capacity. On the other hand, esterification of the terminal phosphate of GTP with a nucleoside or a methyl group led to a total loss of activating ability: dinucleoside triphosphates and the gamma-phosphate methyl ester of GTP acted as competitive antagonists of the activators. alpha,beta- and beta,gamma-imino and -methylene derivatives of both ATP and GTP stimulated the enzymatic activity, suggesting that activation can occur without covalent transfer either of the terminal phosphate or pyrophosphate, or of the nucleotidyl residue. The stimulatory effect of the beta,gamma-imino derivatives of ATP and GTP was not additive. The inhibition constants obtained with gamma-phosphate esters of GTP were the same for either one of the two imino analogs. It is concluded that adenosine and guanosine derivatives bind to the same domain of the enzyme. It is also postulated that activators may interact with the enzyme or with a regulatory protein at two locations, a binding site for the nucleoside moiety and a "functional" site for the pyrophosphate residue.