The usual regimens of captopril--twice or thrice daily administration--are based on the duration of the decrease in plasma angiotensin II induced by captopril. In a study performed to evaluate the hypotensive effect of once daily captopril, 13 white patients with mild-to-severe uncomplicated hypertension were treated with one tablet of captopril 100 mg daily, taken 1-1.5 h before lunch, for 8 weeks. Arterial blood pressure was measured weekly, 22-23.5 h after medication. The patients' diet contained no more than 120 mmol/day of sodium. In the first week supine blood pressure fell from 210 +/- 3/128 +/- 4 (mean +/- SEM) to 179 +/- 5/116 +/- 5 mm Hg (p less than 0.001/p less than 0.01 compared with pretreatment). After the large decrease in the first week changes in systolic and diastolic pressures tailed off; they tended to fall towards stable values that would be maintained on prolonged treatment. At the end of the eighth week the supine values were 155 +/- 3/104 +/- 3 (p less than 0.001/p less than 0.001). Changes in erect blood pressure paralleled those in the supine posture. No side effects were detected. These results confirm that captopril is efficacious when given alone to patients with essential hypertension who are taking a low sodium diet. Blood pressures were not, however, reduced to normotensive levels. Captopril's hypotensive effect in once daily administration appears to be independent of its effects on circulating angiotensin II. Captopril alone 100 mg/day is thus indicated in essential hypertension and should be prescribed once daily to obtain the best possible compliance.