I.v. injection of a saline extract of lyophilized rat atrial myocardium into anesthetized rats caused a massive diuresis and natriuresis of short duration. There was also a significant increase in potassium excretion and a fall in urine osmolality. There were no significant changes in solute-free water reabsorption, glomerular filtration rate, renal plasma flow or filtration fraction as measured by conventional clearance methods. Mean arterial pressure was significantly reduced during the natriuretic response. The natriuretic and diuretic response was not blocked by inhibition of prostaglandin synthesis with aspirin and indomethacin. Infusion of a small amount of atrial extract directly into the left kidney resulted in a natriuretic and diuretic response which was predominantly localized to the left kidney. Apparently, saline extracts of rat atrial myocardium contain a potent natriuretic substance which acts directly on kidneys by a mechanism which does not depend on increased synthesis of prostaglandins.