In 33 patients with heart failure (NYHA II-III) 24-h blood pressure was examined during the titration of two ACE-inhibitors. Blood pressure was measured by the oscillometric method using the blood pressure monitor 90202 from SpaceLabs, Inc. The measurements were taken from 06.00 to 22.00 hours every 20 min, and from 22.00 to 06.00 hours every 1 h. All patients received an additional therapy, either with captopril (group 1, n = 17) or enalapril (group 2, n = 16) in random order. Serum-electrolytes, serum-creatinine, and plasma-renin activity were measured before and during therapy with both ACE-inhibitors. 24-h blood pressure measurements were taken before and on the first and fifth day of the treatment with ACE-inhibitors. The groups did not different in respect to the degree of heart failure, the concomitant medication, or the 24-h profiles of blood pressure and heart rate. The mean initial doses of captopril was 9.2 +/- 1.2 mg. Each patient of group 2 received an initial dose of 2.5 mg enalapril. The maximal decrease of diastolic blood pressure occurred after 1 h in group 1 and after 4 h in group 2 and was similar in both groups (8 vs, 7 mmHg). The 24-h blood pressure values on day 5 were consistently below the pretreatment values (p less than 0.005). Heart rate was not affected by either ACE-inhibitor. The groups did not differ significantly during ACE-inhibition in their 24-h blood pressure and heart rate profiles. Before treatment, serum-sodium, -potassium and -creatinine were within the normal range.(ABSTRACT TRUNCATED AT 250 WORDS)