An investigation of the therapeutic effect of a once daily dose of captopril was carried out on 19 patients with essential hypertension. Captopril was administered at a dose of 50 mg/day for 2 weeks. If blood pressure was not reduced to less than 160/95 mm Hg in that time, the dose was increased to 100 mg/day for a further 2 weeks (5 patients). If blood pressure remained high, 50 mg chlorthalidone/day was added (4 patients). Only 1 patient was not controlled. The drugs were administered in the morning. Blood pressure and serum angiotensin converting enzyme activity (SACE) were measured 24 hours after the last dose, and patients were followed up to 3 months. The basal SACE decreased 24 hours after the first captopril dose but 3 months later, when blood pressure was controlled, the mean level was back to baseline values. Our results show that once-daily captopril is efficacious in hypertension and its hypotensive effect appears to be independent of ACE inhibition.