The hypotensive effect of captopril 50 mg twice daily and of captopril 50 mg + hydrochlorothiazide (HCTZ) 25 mg once daily was studied in 12 patients with mild to moderate essential hypertension, whose blood pressure was not normalized by captopril 25 mg twice daily alone. Both captopril 50 mg twice daily and captopril 50 mg + HCTZ 25 mg once daily caused a significant reduction of outpatient blood pressures as compared with placebo (P less than 0.001). Captopril 50 mg + HCTZ 25 mg once daily also reduced outpatient blood pressures significantly when compared with captopril 25 mg twice daily (P less than 0.01). Both captopril 50 mg twice daily and captopril 50 mg + HCTZ 25 mg once daily significantly reduced 24 h blood pressure (P less than 0.001) without disturbance of its normal circadian rhythm. This effect was more pronounced while on captopril + HCTZ. Captopril 50 mg + HCTZ 25 mg once daily normalizes 24 h blood pressure in most patients with mild to moderate essential hypertension, whose blood pressure is not controlled by captopril 25 mg twice daily alone.