Ticlopidine, an inhibitor of platelet activation enhances the antiaggregating effect of prostaglandins (PG) E1 and I2 when applied to rat whole blood or platelet-rich plasma. Aggregation by ADP was inhibited to a similar extent when evaluated on whole blood and on PRP of ticlopidine-treated rats showing that circulating PGs are probably not directly responsible for the antiaggregating effects of the drug. When platelets collected from ticlopidine-treated rats were separated from their plasma, they became largely refractory to ADP. PGE2 was equally effective in potentiating platelet aggregation of washed platelets of treated and untreated animals, but failed to fully restore the impaired responsiveness to ADP of washed platelets from ticlopidine-treated rats. Our results indicate that ticlopidine does not prevent platelet aggregation by enhancing the antiaggregating effect of circulating PGs, and that ticlopidine or a metabolite, interact directly with the rat platelet, reducing its responsiveness to ADP.