Flunarizine, a calcium (Ca2+)-entry blocker, selective for vascular tissues, inhibits in a concn-dependent way the contraction of isolated rat caudal artery preparations induced by mediators derived from thrombin-stimulated rat platelets. This inhibition is slow in onset and is of prolonged duration. Specific measurements and pharmacological analysis show 5-hydroxytryptamine (5HT) and thromboxane A2 (TXA2) to be the main mediators involved. Comparison with exogenously added agonists shows amplification between 5HT and TXA2 at the level of the vascular smooth muscle cells. Combined treatment with ketanserin, a selective 5HT2 receptor antagonist, and suprofen, a fatty acid cyclo-oxygenase inhibitor, shows that flunarizine inhibits the 5HT-induced as well as the prostaglandin-induced components of the contraction. The compound does not affect the release of 5HT from the platelet and does not interfere with the biosynthesis of TXA2 from endogenous platelet arachidonic acid; it reduces the amounts of TXB2 and HHT and increases the production of HETE from exogenous [14C]arachidonic acid by washed rat platelets.