Administration of a newly discovered second atrial peptide, iso-atrial natriuretic peptide (or iso-rANP(1-45) for the rat), caused hypotension, decreased heart rate, diuresis, and increased renal excretion of Na+, K+, and Cl- in the anesthetized rat. Bolus injections of chemically synthetic iso-rANP(1-45) had circulatory and diuretic activity equal to or greater than rANP(99-126) but, at low doses, a lesser effect on renal electrolyte excretion. The synthetic peptide fragment, iso-rANP(17-45), analogous in structure to rANP(99-126), had attenuated activity on the circulation, and at low doses, attenuated activity on the kidney. At higher doses, where renal responses to rANP(99-126) were less (downside of a biphasic response), both iso-rANP(1-45) and (17-45) had greater effects on water and electrolyte excretion than rANP(99-126). Injections of iso-rANP(1-45) and (17-45) increased hematocrit, whereas rANP(99-126) did not; furthermore, unlike rANP(99-126), iso-rANP did not affect arterial plasma Na+ concentration. The heart produces at least two genetically different atrial natriuretic peptides which affect the circulation and salt and water balance.