Tetrodotoxin given to cats in doses of 1 to 10 μg/kg iv caused profound cardiovascular depression characterized by decreases in systolic and diastolic blood pressures, pulse pressure, heart rate, force of myocardial contraction, cardiac output and peripheral resistance. These effects were associated with marked inhibition of the responses to stimulation of nerves to the adrenal medulla and the sympathetic nerves to heart and blood vessels. Sinus bradycardia due to toxin did not occur after acute cardiac sympathetic denervation. In isolated sympathetic nerve-right atrium preparations, tetrodotoxin at 1 to 2 × 10−8g/ml abolished the positive inotropic and chronotropic responses to nerve stimulation without effect on spontaneous rate or force, or transmitter release by tyramine. Toxin-produced vasodilation in perfused hindlimbs was associated with marked inhibition of the normal vasoconstrictor responses to sympathetic vasomotor nerve stimulation. Vasodilation was not due to a direct action on vascular smooth muscle, since it was not seen after α-receptor blockade with phenoxybenzamine, after sympathetic denervation, or after blockade of adrenergic transmitter release by guanethidine or β-TM 10 accompanied by adrenalectomy. Vasodilators such as acetylcholine, histamine and isoproterenol consistently produced additional vasodilation under these conditions. The hypotensive effect of the toxin was also observed in spinal cats. It is concluded that the potent hypotensive action of tetrodotoxin, in the cat, is primarily due to blockade of conduction in peripheral sympathetic nerves and nerve fibers innervating the adrenal medulla.