The possibility of a direct and casual relationship between various parameters of the adenosine 3',5'-cyclic monophosphate (cAMP) system, e.g., the level of cAMP, adenyl cyclase activity and phosphodiesterase activity, and platelet aggregation was studied by measuring the effects of various environmental conditions, as well as metabolic inhibitors, aggregating agents and aggregation inhibitors upon these parameters. A competitive binding technique using 3H labeled cAMP was used to determine the level of cAMP in intact platelets, and a high performance liquid chromatographic method was developed to measure the adenyl cyclase and phosphodiesterase activities in the platelet membrane fraction. Although the availability of substrate adenosine triphosphate (ATP) correlated well with the amount of cAMP produced, and in turn the availability of cAMP seemed to have a direct effect upon the reversibility of shape changes induced by the various stimuli, the only effect upon aggregation concerned the extent to which it occurred. No direct correlation of the level of cAMP with either the actual inhibition or activation of the aggregation mechanism was observed.