The action of the antibiotic novobiocin on transepithelial Na transport was studied in isolated skins obtained from two different frog species. In Rana esculenta addition of novobiocin to the outer bath (1 mM) resulted in a sustained and reversible stimulation of the short-circuit current, transepithelial potential, and transepithelial conductance. Similar, though more variable and much less pronounced changes were observed in Rana temporaria. In the presence of amiloride (0.1 mM) novobiocin had no effect on any of the investigated transport parameters and all novobiocin induced changes were fully reversed when amiloride was given subsequently. At reduced external Na concentration or low pH the action of novobiocin was found to be greatly attenuated. In the presence of novobiocin an increased affinity to amiloride and a linearization of the transepithelial current-voltage relationship was observed. The results are consistent with the view that novobiocin increases the Na permeability of the outer membrane, possibly by an attenuation of an Na self-inhibition mechanism. In addition, the driving force of transepithelial Na transport was estimated by means of novobiocin. Several different methods were employed, providing varying results. As shown in an Appendix, for the most part the discrepancies can be explained by changes in the intracellular Na and K concentration. In some cases, novobiocin induced large secondary increases in the skin conductance which can be referred to an increased Cl permeability.