The stimulation of DNA synthesis in quiescent, density-arrested BALB/c-3T3 cells by platelet-derived growth factor in plasma-supplemented medium was inhibited by the presence of isobutylmethylxanthine (IBMX) and cholera toxin, although neither IBMX or cholera toxin when used alone inhibited the stimulation of DNA synthesis. The cells were reversibly inhibited in mid G1 at a point 6 hr prior to the initiation of DNA synthesis. The inhibition of cell cycle traverse was associated with a 10-15 fold increase in cellular cyclic AMP concentration over basal levels. The reversal of this inhibition by removal of IBMX was correlated with a dramatic decrease in cyclic AMP levels. The traverse of G1 and the initiation of DNA synthesis after release from the cholera toxin and IBMX inhibition was dependent on the presence of plasma in the medium. Either somatomedin C (10-20 ng/ml) or insulin (10(-6)-10(-5) M) completely replaced the plasma requirement for late G1 progression and entry into S phase. Once the inhibited cells were released from the IBMX and cholera toxin block a subsequent increase in cyclic AMP did not prevent entry into S phase. The presence of cholera toxin alone inhibited the stimulation of human dermal fibroblasts. The elevation of intracellular cyclic AMP levels in the human dermal fibroblasts by cholera toxin was two to three fold greater than that found in the BALB/c-3T3 cells in the presence of cholera toxin and the IBMX.