Short-circuit current (SCC) techniques were used to monitor the effects of various diuretic agents on Na+ transport in isolated frog skin, a model for the late distal tubule and the collecting duct of the mammalian kidney. Acetazolamide, hydrochlorothiazide, torasemide, and ethacrynic acid did not affect sodium transport (as indicated by the SCC) or transepithelial electrical resistance when added either to the apical (outer) or to the inner (basolateral, corial) bathing solution of the tissue. However, Na+ transport was sensitive to amiloride, the triamterene derivate dimethylamino-hydroxypropoxytriamterene (RPH 2823), and to furosemide. Whereas apical amiloride, and RPH 2823 induced a dose-dependent decrease in SCC and increase in transepithelial electrical resistance, apical furosemide resulted in a dose-dependent increase in SCC and a decrease in electrical resistance. None of the three diuretic agents caused a significant change in SCC when applied to the inner bathing Ringer's solution. The small furosemide-induced decrease in resistance compared with the huge increase in SCC suggests that furosemide affects Cl- permeability as well as Na+ permeability. Evidence for this notion was achieved by the following findings: The decrease in resistance after furosemide was more pronounced in tissues bathed in Cl(-)-free solutions compared with Cl(-)-containing solutions. n contrast, SCC stimulation by apical furosemide is Cl(-)-ion independent, but strongly Na+-ion dependent. SCC stimulation by furosemide is amiloride-sensitive. With respect to the onset, locus, and reversibility of action, it seems reasonable to assume that amiloride, RPH 2823, and furosemide all influence transepithelial Na+ transport by interacting with the Na+ channel or a regulator site of it within the apical membrane. The stoichiometry of the amiloride (RPH 2823)-receptor site interaction revealed Hill-coefficient(s) of less than 1, indicating a negative cooperativity among the receptor sites. The interaction between Na+ ions and amiloride or RPH 2823 displayed mixed competitive-noncompetitive inhibition. Taken together, these results support the hypothesis that amiloride and Na+ as well as RPH 2823 and Na+ may act at different loci on the apical entry mechanism in Rana esculenta skin.