The objectives of this study were to evaluate baseline sympathetic nerve activity as well as the mechanisms of sympathetic overactivity in mild chronic renal insufficiency hypertension. Seven hypertensives with mild renal insufficiency, seven hypertensives with normal renal function and seven normotensives, age and weight-matched were studied on one session to evaluate baseline muscle sympathetic nerve activity measured in the peroneal nerve. The mild renal insufficiency hypertensives and the hypertensives with normal renal function were also studied to evaluate arterial baroreflex control of muscle sympathetic nerve activity assessed by increasing and decreasing blood pressure through continuous infusion of phenylephrine and sodium nitroprusside respectively. Baseline muscle sympathetic nerve activity was significantly higher in mild renal insufficiency hypertensives (34 bursts/min) when com-