In a randomized, double-blind parallel group study the 24-hour hypotensive effect of piretanide and its influence on biochemical variables were compared with those of placebo in patients with mild to moderate essential hypertension. Sixty patients entered the study, all of whom met the inclusion criteria (RRdiast between 95 and 120 mmHg). There was no drop-out during the study, so that the results of all 60 patients were statistically analysed. Piretanide produced a significant reduction of both systolic and diastolic blood pressure over 24 hours which was evident at four weeks and was maintained and further enhanced over the ensuing trial period. A mean maximal fall (at 12 weeks) of 10.7% (BPdiast supine) was observed. Placebo tablets did not produce any clinically relevant changes in systolic blood pressure, whereas a slight decrease was seen in diastolic blood pressure. This blood pressure reduction was significantly less in the placebo group than in the piretanide group at the end of the study (weeks 10 and 12). Dose doubling was needed in 13 of the 30 patients in the piretanide group, whereas as many as 20 out of 30 patients needed dose doubling in the placebo group. Pulse rate did not change relevantly during the trial in either group. A slight reduction in body weight was observed in the piretanide group. The mean values of serum potassium and sodium showed a slight decrease but remained within the normal range during the study period. A small increase in serum phosphorus was noted. None of these changes required any specific measures.(ABSTRACT TRUNCATED AT 250 WORDS)