Acetylsalicylic acid (ASA) did not prevent the arrhythmias induced in rats by a bolus intravenous injection of ADP, an effect independent of platelets. ASA was also unable to protect the rats from the platelet-mediated cardiovascular effects and myocardial ischaemia induced by a prolonged ADP infusion. It is suggested that the release of vasoactive substances inhibited by ASA does not play a major role in the cardiovascular effects of ADP in rats.