The effect of captopril 25 to 50 mg twice daily on 24 h blood pressures was compared with placebo in 14 patients with essential hypertension. Captopril was started in a dose of 25 mg b.d. in all patients. This dose was increased to 50 mg b.d. in those patients whose blood pressure was not normalized after 4 weeks (group II, n = 5), while in the others the same dose was continued (group I, n = 9). Blood pressure was measured at the end of the placebo period and after 8 weeks captopril, both in the office with a random zero sphygmomanometer and ambulant at home with a portable automated non-invasive blood pressure measuring device (ICR). In comparing the ICR measurements with those made with a standard mercury sphygmomanometer good agreement between the two methods for both diastolic (r = 0.94, n = 110) and systolic (r = 0.96, n = 110) blood pressure was found. In group I office blood pressures (mean +/- s.e.m.) decreased from 166 +/- 7/100 +/- 3 mm Hg to 144 +/- 4/87 +/- 2 mm Hg (P less than 0.001). In group II these reductions were from 165 +/- 7/104 +/- 2 mm Hg to 152 +/- 5/90 +/- 4 mm Hg (P less than 0.05). During ambulatory monitoring, blood pressure was shown to have a circadian rhythm both on placebo and on captopril. Captopril 25 mg b.d. lowered blood pressure throughout the 24 h observation period, although these reductions were smaller at night than during the day. In group II comparable effects of captopril 50 mg b.d. were demonstrated. Captopril 25 to 50 mg twice daily adequately controls blood pressure in most patients with mild to moderate essential hypertension.