In rat platelets, basal cAMP level did not vary significantly during ADP-induced aggregation. In the same conditions, no variation in the cAMP content was observed in platelets from rats treated with clopidogrel, whereas ADP-induced aggregation was totally inhibited. ADP decreased cAMP level in control prostacyclin- or forskolin-stimulated platelets whereas, in treated platelets, adenylyl cyclase down-regulation was strongly inhibited. SQ 22536 (500 microM), an inhibitor of adenylyl cyclase, strongly reduced the cAMP content of both control and treated platelets but did not reverse the anti-aggregating activity of clopidogrel, showing that inhibition of ADP-induced adenylyl cyclase down-regulation in treated platelets was not responsible for the anti-aggregating effect of clopidogrel. Similar results were obtained in rabbit platelets. These results therefore demonstrate that cAMP is not an important second messenger for ADP-induced platelet aggregation and suggest that another activating pathway, linked to the low affinity ADP receptor present on the platelet surface might be involved in the aggregation process.