To determine if there would be a decrease in blood pressure after exercise in patients with borderline hypertension and if this decrease would be accompanied by a decrease in sympathetic nerve activity to muscle, we recorded multifiber postganglionic muscle sympathetic activity from the peroneal nerve at rest in nine men with borderline hypertension (age 25 ± 1 years, mean ±SEM) before and 60 minutes after 45 minutes of submaximal treadmill exercise. In addition, responses to a cold pressor test, handgrip, and the Valsalva maneuver were recorded before and after exercise. Four subjects were also studied before and after 'sham' exercise. Sham exercise had no effect on blood pressure or sympathetic nerve activity whereas resting systolic blood pressure was lower after treadmill exercise in seven subjects (from 136 ± 4 before to 123 ± 2 mm Hg 60 minutes after exercise; p < 0.01). Sixty minutes after exercise, sympathetic nerve activity was lower in all seven subjects (from 19 ± 2 to 11 ± 2 bursts/min, p < 0.015; or from 27 ± 3 to 14 ± 2 bursts/100 heartbeats, p < 0.005) but was slightly increased in the two subjects without postexercise hypotension. Heart rate and pressor and sympathoneural responses to the cold pressor test, handgrip, and the Valsalva maneuver were not altered by prior exercise. When nitroprusside was infused in five subjects to produce a reduction in systolic blood pressure similar to that seen 60 minutes after exercise, this drug increased sympathetic discharge from 37 ± 6 to 57 ± 4 bursts/100 heartbeats (p < 0.001). These observations demonstrate 1) that rhythmic exercise can lower blood pressure in men with borderline hypertension and 2) that postexercise hypotension is associated with a decrease, rather than a reflex increase, in sympathetic discharge to muscle. Postexercise hypotension may be mediated in part by inhibition of sympathetic nerve activity.