In a single-blind, randomized cross-over study the bioavailability of Spironothiazid (50 mg spironolactone, 50 mg hydrochlorothiazide) was investigated in six healthy male volunteers by comparing the same dose of the well-established drugs Aldactone dragees, 50 mg, and Esidrex tablets, 25 mg. After 6 days treatment with 2 X 1 Spironothiazid or 2 X 1 Aldactone dragees, 50 mg, plus Esidrex tablets, 25 mg daly, orally, the serum concentrations and the cumulative renal excretion of the active substances were measured after the morning dose on the 7th day of the study. Canrenone (the main metabolite of spironolactone) was measured fluorimetrically, hydrochlorothiazide was determined by gas chromatography. There was no statistically significant difference between the investigated drugs, neither in the areas under the curves nor in the cumulative renal excretion, thus indicating an identical bioavailability.