The stem, wood and bark of Uncaria kawakamii contain the two isomeric alkaloids, Uncarine A and Uncarine B, the highest proportion of alkaloids (1.48%) occurring in the bark. Uncarine A caused a marked decrease of systolic and diastolic pressures in renal or spontaneously hypertensive conscious rats, but not in normotensive rats. During the period of hypotensive action, the heart rate was increased. No effect on the noradrenaline content of the heart or on the pressor responses to noradrenaline, angiotensin II, or sympathetic stimulation was observed. The concentrations of plasma noradrenaline and adrenaline in the spontaneously hypertensive rat were markedly increased when the blood pressure was lowered. Uncarine A apparently reduces the blood pressure of hypertensive rats by a mechanism different from that of currently used antihypertensive agents. The site of action could be the central nervous system.