The sulphonamide-free diuretic Bay g 2821 was tested under standardized conditions 43 oedema-free volunteers. The threshold dose was 10 mg (0.14 mg/kg). The doseresponse curve was practically linear for doses up to 80 mg. The excretion of sodium and chloride was very high, and that of potassium, magnesium and calcium markedly lower. Itarenal haemodynamic investigations showed that the inulin clearance was unchanged, and the PAH clearance increased slightly. The tubular reabsorption of sodium and chloride was inhibited. Side-effects after a single oral dose did not occur.