There is evidence that platelet activation in the coronary circulation may be important in the pathogenesis of myocardial ischemia. Since organic nitrate vasodilators are commonly used in coronary artery disease, we have studied the in vitro effects of these drugs on platelet function. Nitroglycerin, isosorbide dinitrate, and their biotransformation product, inorganic nitrite, inhibited platelet aggregation with collagen, epinephrine, arachidonate, and ionophore, and blocked both primary and secondary aggregation in response to ADP. Nitroglycerin was studied in more detail. Its inhibitory effect was reversible and not dependent on external calcium concentration. It inhibited arachidonic acid oxygenation as measured by the arachidonate- induced oxygen burst and malonaldehyde production. These effects were not due to an increase in intracellular cyclic AMP. This unusual generalized inhibition of platelet function by nitroglycerin possibly contributes to its beneficial effect in myocardial ischemia in part by attenuating platelet reactivity in the coronary circulation.