In unanesthetized dogs within 20 min after intravenous administration of 0.05 mg/kg of mezaton an increase of diuresis, the rate of glomerular filtration, effective renal plasma flow, filtration fraction, sodium and potassium excretion by the kidneys was observed. The dose of 0.5 mg/kg of mezaton produced the opposite changes. In the experiments on frogs mezaton administration in a dose of 200 mg/kg exerted the positive inotropic effect.