Atrial natriuretic peptide (ANP) can be extracted from rat hearts, and is found to increase fluid excretion by the kidneys when injected into test animals. The mechanism of ANP action is still unclear. ANP may reduce sodium reabsorption in the renal tubules, but it is also known that it increases the rate of glomerular filtration in the kidney, and relaxes preparations of smooth muscle, including one made from arteries that supply the kidney. To clarify its mode of action, we have studied directly the effects of semi-purified and synthetic ANP on blood vessels in the kidney of anaesthetized rats. We found that ANP causes a vasodilatation of the blood vessels which supply the glomeruli and a vasoconstriction of the arterioles which drain them. This substantiates the finding that increased filtration pressure participates in the natriuretic response.