We studied the effects of triamterene (TA) and its phase I and phase II metabolites, p-hydroxytriamterene (OH-TA) and p-hydroxytriamterene sulfuric acid ester (OH-TA ester), on sodium transport in the isolated frog skin. While TA applied to the inside (corial side) surface had no effects on potential difference (PD) and short-circuit current (SCC), TA, OH-TA, and OH-TA ester at a concentration of 10^–5 mol/l significantly decreased SCC by 19, 24 and 16%, respectively, when added to the solution bathing the outside (epithelial side) surface of the skin. In vasopressin-treated skins TA was more effective than OH-TA or OH-TA ester. In aldosterone-treated skins all compounds significantly suppressed SCC, the strongest effect was again exerted by TA. Only minor and transient changes in PD were noted. Inhibition of sodium transport was rapidly reversible when skins were washed with fresh Ringer solution. Thus, OH-TA and OH-TA ester possess qualitatively similar pharmacological activity as TA. The quantitatively smaller effects of OH-TA and OH-TA ester in the hormone-stimulated skin as compared to TA agrees well with their relative natriuretic potency observed in vivo.