Mixidine is negative chronotropic agent patented by McNeil Laboratories. Mixidine produced a dose-related decrease in heart rate elevated reflexly by aminophylline, by beta-adrenergic stimulation induced by isoproterenol, by sympathetic nerve stimulation and by intravenous infusion of glucagon. Mixidineattenuated the increase in contractile force produced by sympathetic nerve stimulation but not that induced by isoproterenol. The compound antagonized the increase in the rate of isolated guinea-pig atria induced by both isoproterenol and histamine. In the conscious dog, Mixidine caused no decrease in resting heart rate, mean arterial pressure and cardiac output. It reduced atropine-induced sinus tachycardia as well as that induced by treadmill exercise.