The long-term effect of exercise on blood-pressure (BP) was assessed in 13 untreated patients with essential hypertension. After a 6-week run-in period the levels of activity studied were sedentary, 45 min bicycling at 60-70% of maximum work capacity (Wmax) three times per week (3/week), and 45 min bicycling seven times per week (7/week), each for 4 weeks. The order differed between subjects in accordance with a Latin square. Supine BP, 48 h after each phase, averaged 148/99 mm Hg in the run-in and 143/96 mm Hg in the sedentary phase; it fell below values in the sedentary phase by 11/9 mm Hg with 3/week exercise, and by 16/11 mm Hg with 7/week exercise (both p less than 0.01). With increasing activity total peripheral resistance fell and the cardiac index rose. Plasma noradrenaline concentration fell below values in the sedentary phase by 21% and 33% after 3/week and 7/week exercise. Bodyweight and 24 h sodium excretion remained constant. Moderate regular exercise lowers BP and seems to be an important non-pharmacological method of treating hypertension.