Guanazodine is a new antihypertensive drug. Guanazodine caused a sustained decrease in the systemic blood pressure of spontaneously hypertensive rats, renal hypertensive dogs and normal cats. No tachyphylaxis developed when the drug was administered orally. The heart rate decreased. Guanazodine relaxed the cat nictitating membrane, attenuated the positive chronotropic response to sympathetic nerve stimulation in anesthetized dogs and in isolated rabbit aorta to transmural electrical stimulation. Guanazodine potentiated the pressor response to noradrenaline but attenuated the response to tyramine in anesthetized cats. It may be concluded that the hypotensive effect of guanazodine is related to adrenergic neuron blocking action, the noradrenaline-depleting action in peripheral tissues is similar to the effect of guanethidine and bethanidine. However, this drug is less potent than guanethidine. Toxicity and side effects appear to be less with guanazodine than with guanethidine and bethanidine.