To determine whether the baroreflex control of sympathetic nerve activity is altered in patients with essential hypertension, muscle sympathetic nerve activity (MSNA) was recorded microneurographically from the tibial nerves of 23 normotensive subjects and 23 patients with essential hypertension. When phenylephrine (2 micrograms/kg) was injected intravenously, although the pressor response of mean arterial blood pressure (MAP) was significantly enhanced in the hypertensives as compared with the normotensives, the reflex decrease in MSNA was significantly smaller in the hypertensives. Furthermore, the baroreflex slope for MSNA, used as an index of baroreflex sensitivity and calculated by relating the change in MSNA to the change in MAP, was significantly less in the hypertensives. Following the injection of nitroglycerin (2 micrograms/kg), there were no significant differences between the normotensives and hypertensives in the depressor response, the reflex increase in MSNA or the baroreflex slope for MSNA. These observations suggest that the baroreflex change in sympathetic nerve activity is reduced during phenylephrine-induced blood pressure elevation but not during nitroglycerin-induced hypotension in the hypertensives, and that the blunted response of sympathetic nerve activity occurring during hypertension in these hypertensive patients may underlie the maintenance of high blood pressure in essential hypertension.