A method is described that allows perfusion of the inner medullary collecting duct (IMCD) of the rat kidney in situ and in vivo. Fine polyethylene catheters connected to a microperfusion pump were inserted into collecting ducts via the openings at the exposed papilla tip. Perfusate contained 22Na as well as [3H]inulin. During perfusion at 30 nl/min, urine was simultaneously collected. A decrease in the Na-to-inulin concentration ratio in the urinary sample, compared with the perfusate, was taken as indicating unidirectional efflux of Na from the perfused duct system. The effects of luminal amiloride (2 X 10(-4) M) or atrial natriuretic factor (ANF, 10(-8) M) were studied. Compared with control perfusions, both agonists reduced Na efflux from the IMCD to approximately 50%, indicating luminal sites of action. Combination of amiloride and ANF at their respective concentrations had no further effect. The lack of statistically significant additivity suggests, but does not prove, that ANF, administered from the luminal side, is able to block amiloride-sensitive Na channels in the apical membrane of IMCD cells.