Platelet aggregation induced by ADP in canine platelet-rich plasma was inhibited by volatile anesthetics. The partial pressures at which platelet aggregation was inhibited 50 per cent correlated closely with the pressures used clinically: methoxyflurane, 3.5 torr; halothane, 7.5 torr; diethyl ether, 36 torr; cyclopropane, 120 torr. Nitrous oxide at 640 torr inhibited platelet aggregation 34 per cent.