This study examined the effects of angiotensin II (Ang II) on blood pressure, heart rate and muscle sympathetic nerve activity in patients with essential hypertension and normotensive subjects. Muscle sympathetic nerve activity was measured by a microneurographic method. Graded doses of phenylephrine (0.5 and 1.0 g/kg per min) or Ang II (5, 10 and 20 ng/kg per min) were intravenously infused into supine subjects for 10 min at 30-min intervals. Although the pressor responses to Ang II and phenylephrine were enhanced in the hypertensives compared with the normotensives (both P less than 0.05), the reduction in muscle sympathetic nerve activity during infusion of phenylephrine and Ang II and the reduction in the heart rate during infusion of phenylephrine were significantly smaller in the hypertensives than in the normotensives. The decrease in the heart rate in response to Ang II was not significant in either group. Furthermore, in the two groups, the decreases in muscle sympathetic nerve activity and the heart rate were significantly smaller during infusion of Ang II than during infusion of phenylephrine. These results suggest that angiotensin II impairs the baroreflex function in patients with essential hypertension and in normotensive subjects, and that the blunted response of sympathetic nerve activity and the heart rate observed in the hypertensives may underlie the maintenance of high blood pressure.