Muzolimine, a new saliuretic, has been shown to combine high ceiling and long-acting effects in animal experiments. This study was designed to examine whether this desirable combination of effects, which up until the present time has not been incorporated into any substance also occurs during patient treatment. Fifty-three patients with mild essential hypertension (WHO groups I and II) in three medical centers were treated with either muzolimine or indapamide, which served as the reference preparation, in a randomised, double-blind study. After a two week run-in phase during which the patients received placebo, half of the patients received 20 mg muzolimine and the other half 2.5 mg indapamide once daily. Eight weeks of therapy were followed by a 2 week follow-up phase, during which placebo was dispensed. During the trial period a weekly clinical examination was performed including the measurement of blood pressure, pulse, hematocrit, electrolytes, uric acid, glucose, creatinine and lipid status. An electrocardiogram and Schellongtest were conducted every two weeks. Patients were instructed to keep a diary in which they were to note drug related complaints. Statistical analysis was carried out using the Pratt-Wilcoxon Pair Test. Differences were judged significant at the 5% level. Both muzolimine and indapamide were tolerated well with minimal side effects, which however, did not make it necessary to discontinue treatment. Both preparations induced mild blood pressure reductions of approximately 10 and 5 mmHg for the systolic and diastolic blood pressure, respectively. Two weeks after cessation of muzolimine treatment neither systolic nor diastolic blood pressure showed any significant difference to values achieved during the treatment phase.(ABSTRACT TRUNCATED AT 250 WORDS)