In 12 essential hypertensive patients insufficiently controlled by treatment with 50 mg hydrochlorothiazide, the blood pressure lowering effect of the addition of 25 mg captopril once daily was examined. Twenty four hour ambulatory blood pressure monitoring was performed using portable automatic recording devices before treatment, after 4 weeks of treatment with hydrochlorothiazide alone, and again after 4 weeks of combined treatment with captopril. The combined treatment yielded a marked additional blood pressure reduction in comparison with the previous medication of the diuretic alone. The additional antihypertensive effect of captopril lasted 24 hours. During the daytime between 08.00 h and 20.00 h the mean blood pressure was 158 +/- 13/100 +/- 8 mmHg before treatment. After treatment with hydrochlorothiazide alone it was 151 +/- 11/93 +/- 8 mmHg and after hydrochlorothiazide and captopril it was 140 +/- 13/88 +/- 9 mmHg. Our results indicate that the addition of only 25 mg captopril once a day to a diuretic may result in a marked additional blood pressure reduction or normalization in cases of insufficient control by the diuretic alone.