Equine platelet aggregation was stimulated by collagen fibres or platelet-activating factor. The action of both ligands was blocked by forskolin or prostaglandin E1agents which are known to activate adenylate cyclase. Equine platelet membranes were found to contain adenylate cyclase activity which was inhibited in dose-dependent fashion by both collagen and platelet-activating factor. Platelet-activating factor-induced inhibition was antagonised by WEB2086.