The shape of the time-course of cyclic AMP formation by intact human platelets in response to the stable prostaglandin I2 analogue iloprost varied with the concentration of the prostaglandin. At low concentrations of iloprost, the time-course showed a rise to a plateau with little subsequent decrease in cyclic AMP level. At high concentrations of iloprost, the initial rate of cyclic AMP formation was more rapid than at low concentrations, but the curves showed a marked time-dependent fall in cyclic AMP level to values below those observed at lower prostaglandin concentration. By contrast, PGE1 gave a rise and marked fall in cyclic AMP level at all concentrations of the prostaglandin and the curves did not cross. The time- and concentration-dependent fall in cyclic AMP level in response to iloprost was still apparent in the presence of phosphodiesterase inhibitors, indicating that inhibition of adenylate cyclase, rather than activation of cyclic AMP phosphodiesterases, was responsible for the fall in cyclic AMP level. Activators of protein kinase C, which phosphorylates platelet Ni and impairs its function, abolished the time-dependent fall in cyclic AMP level, indicating that Ni may be involved in prostaglandin-induced inhibition of adenylate cyclase. Time-courses were analyzed using an equation derived by Barber et al. (Adv. Cyc. Nuc. Res. 9, 507-516 (1978)) to yield rate constants for activation and inhibition of adenylate cyclase. Because of the difference in prostaglandin dependence of the activation and inhibition rate constants we propose that activation of adenylate cyclase in platelets is mediated by a rapid-acting stimulatory receptor, while time-dependent inhibition (desensitization) is mediated through a separate, slow-acting inhibitory receptor. The stimulatory receptor has an affinity for prostaglandin greater than the putative inhibitory receptor in the case of iloprost (as well as PGI2 and PGD2), and a lower affinity than the inhibitory receptor in the case of PGE1 (and PGE2). Prostaglandin-induced inhibition may be mediated through Ni.