The effect of additional administration of a low dose of an ACE inhibitor when blood pressure is inadequately controlled by a diuretic alone was investigated in 12 essential hypertensive patients. 24 h blood pressure monitorings were performed with portable automatic recording devices before and after treatment with 50 mg hydrochlorothiazide (HCT) and after additional administration of 25 mg captopril. The frequency of blood pressure readings exceeding 160/95 mm Hg within 24 hours was reduced in 11 of the 12 patients after administration of the ACE inhibitor. Mean blood pressure during daytime was 158 +/- 13 / 100 +/- 8 mm Hg without treatment, 151 +/- 11 / 93 +/- 8 mm Hg during HCT alone and 140 +/- 13 / 88 +/- 9 mm Hg during HCT + captopril. The results show that in a large proportion of hypertensives inadequately controlled by a diuretic alone, the addition of a low dose of an ACE inhibitor (captopril, 25 mg once daily) may normalize the blood pressure.