In anesthetized rats micropuncture and microcatheterization were used to collect tubular fluid from end proximal and distal tubules and from the outer medullary collecting duct. Urine was collected at the papilla tip. Samples were taken from the same sites before and after intravenous injection of atrial tissue extract; rats injected with ventricular extract served as controls. Sodium excretion increased 17-fold after atrial extract, a significantly greater rise than the 3-fold increase after ventricular extract. Clearances of inulin and single nephron filtration rates did not change significantly in either group. Tubular fluid collection results showed a similar reduction (16 to 20%) of proximal fluid and sodium reabsorption in both groups. In the experimental group only, NaCl reabsorption failed to rise in response to increased load in the medullary collecting duct. The resulting fall in fractional reabsorption in the medullary collecting duct accounted for 80% of the natriuresis. We conclude that atrial tissue from rat hearts contains a factor which causes increased renal NaCl excretion by inhibiting transport in the medullary collecting duct.