Carprazidil is oxadiazolopyrimidine derivative patented by pharmaceutical company Hoffmann-La Roche as a direct vasodilator and potent antihypertensive agent. In clinical trials, Carprazidil was evaluated in patients with moderate to severe arterial hypertension. Following the addition of Carprazidil to pre-existing therapy with diuretics and beta-blockers or sympatholytics, blood pressure in most of the patients was normalized within one month. Heart rate was only slightly increased. Orthostatic hypotension was not observed. Weight gain or oedema formation occurred in 14 patients within the first four weeks, but could be controlled satisfactorily by intensified diuretic therapy. After a mean duration of treatment of 2.8 months, plasma volume and plasma and urine sodium were unaltered, and plasma potassium was slightly decreased. Plasma renin activity was doubled, whereas plasma aldosterone concentrations were unaltered. No adverse side effects on hematological parameters, liver or renal function were observed, nor was antinuclear antibody detected.