A single-blind study was carried out in 22 out-patients with mild to moderate hypertension to investigate the effects of piretanide capsules on systolic and diastolic blood pressure during standardized ergometric and isometric exercise. After a 4-week run-in period on placebo, patients received 1 capsule (6 mg) once daily for 6 weeks. If diastolic pressure was normalized (less than 95 mmHg), this dosage was continued for a further 6 weeks; patients whose blood pressure remained high after the initial 6 weeks received 1 capsule twice daily for a further 6 weeks. Significant reductions in blood pressure (9.2% for systolic, 14.6% for diastolic) were measured after 12-weeks' treatment during maximum ergometric work load exercise; during isometric exercise the reductions were 8.1% and 12.4%, respectively. The pulse-pressure product (a parameter indicating myocardial oxygen consumption), calculated for maximum ergometric exercise, was also significantly reduced by 8.8% after 12-weeks' active drug treatment. Serum levels of potassium, creatinine, urea, uric acid and cholesterol showed no clinically relevant changes during treatment. Side-effects definitely or probably associated with piretanide were observed in 13 patients. These, however, were generally mild and did not lead to interruption of piretanide treatment. Transient polyuria was reported by 5 patients during the first 2 weeks of active drug treatment; only 3 patients reported mild polyuria after 12 weeks of active drug treatment. The beneficial effect of piretanide especially on systolic blood pressure during exercise was in marked contrast effect of piretanide especially on systolic blood pressure during exercise was in marked contrast to the lack of such an effect reported during treatment with thiazide and other thiazide-like diuretics.