This study was carried out in association with medical practitioners who were responsible for observing the patients. In a first phase, 58 subjects younger than 70 years with moderate essential hypertension were allotted at random to treatment with either indapamide (2.5 mg per day) or muzolimine (20 mg per day). The double blind-double dummy study lasted for six months and demonstrated that the two drugs were similar in efficacy and tolerance. At the end of this phase 42 patients whose supine diastolic blood pressure fell to below 100 mmHg were selected to continue the trial with 20 mg per day muzolimine in an open long-term study. The survey comprised two periods: the first lasted for six months, the patients being examined every two months, and the second lasted for one year and patients were seen quarterly. However, 8 subjects entered directly into the second period. At each examination standing and supine blood pressure, heart rate and body weight were noted; blood was sampled to allow measurements of serum plasma parameters. Thus 15 patients were treated with muzolimine over two years and 15 over 18 months. Clinically the fall in blood pressure observed initially was maintained throughout the rest of the trial; but 3 patients needed another drug, i.e. central hypotensor (2 cases) and beta-blocker (1 case). Two subjects complained of transitory cramps, but 3 other complications not imputable to the treatment were observed: one myocardial infarction and 2 strokes. Biologically there was no change in mean plasma potassium level, but 3 patients received a potassium supplement.(ABSTRACT TRUNCATED AT 250 WORDS)