Cyclic nucleotide levels were determined in division-synchronized Tetrahymena and the effect of .DELTA.9-tetrahydrocannabinol (THC) on the cyclic nucleotide levels was studied. In non-drug-treated division-synchronized cells, there was no statistically significant variation in the level of cAMP and cGMP during the G2 period, preceding the 1st division. During the free running cell cycle (the interval of be time between the 1st and 2nd synchronous division) the 2-fold increase in the level of cAMP was statistically significant; the variation in the level of cGMP was not statistically significant. THC caused a lowering of cAMP and cGMP levels throughout the 4 h experimental treatment. The suppression of cAMP and cGMP levels altered the cyclic nucleotide pattern of the cell cycle. The cAMP pattern was changed particularly in the G2 period preceding the 1st synchronous division, and immediately after division during the free running cell cycle. THC treatment caused division delays of approximately 8-15 min in the onset of the 1st and 2nd synchronous division. The duration of the free running cell cycle (110-120 min) was unchanged. The suppression of cyclic nucleotide levels resulting from THC treatment is discussed in relation to delays in the division schedule.