The "in vitro" effects of Dilazep on platelet aggregation and arachidonic acid oxidation in platelets and vessel wall were studied. Dilazep exerted a clearcut inhibitory effect on platelet aggregation induced by several stimuli. In addition, the compound completely suppressed malondialdehyde production by platelets elicited by collagen and thrombin, in a dose-dependent fashion. The evaluation of Dilazep effects on the in vitro release of prostacyclin-like material by rat aortic tissue did not provide any indication about an interference of the drug on the generation of endogenous antiaggregatory material.