CO2 production was followed by manometry in starved cell-free extracts of fission yeast stimulated by unstarved cell-free extracts from a synchronous culture. The degree of stimulus, measured by the lag time in CO2 production, varied markedly during the cell cycle, with a maximum for cells at about mitosis and a minimum for septated cells. Similar differences in lag time were found with unstarved extracts of cdc13.117 grown at 37 degrees C and 35 degrees C.