Studies were performed to examine the effect of the atrial natriuretic peptides on the solute gradient of the renal medulla. Experiments were performed in anesthetized male Sprague-Dawley rats given an infusion of either a purified atrial extract (n = 13), atrial natriuretic peptide 8-33 (1 microgram prime followed by 100 ng/min) (n = 8), furosemide (75 micrograms prime and 7.5 micrograms/min, n = 3), or vehicle alone (n = 17). Kidneys were removed 5 min after beginning the infusion, except in four animals, which were infused with atrial extract for 15 min before kidney removal. Both synthetic atrial peptides and an atrial extract result in rapid dissipation of the medullary gradient for urea, sodium, and total solute. No additional loss of solute was apparent at 15 min compared with 5 min. Furosemide caused equally rapid dissipation of the solute gradient at an approximately equidiuretic dose. The fall in tissue concentrations of urea and sodium was attributable to a loss of solute rather than addition of water. Administration of all three agents was associated with an increase in urinary urea excretion, which peaked earlier than the natriuresis. Urinary losses appear to account for a substantial portion of the loss of medullary solute.