The permeability of the skin of a neotenous urodele amphibian, the mudpuppy Necturus maculosus.

1. The permeability of the isolated skin of a neotenous urodele amphibian, the mudpuppy Necturus maculosus, to Na, Cl, urea and water was measured. 2. Unidirectional transcutaneous flux measurements and the action of ouabain and amiloride, showed that there was normally no active Cl or Na transport, nor a Cl/Cl exchange diffusion process. 3. Amphotericin B initiated a transcutaneous potential difference and short‐circuit current, which could be inhibited by ouabain. 4. The short‐circuit current was nearly equivalent to the net Na Transport and this was also inhibited by ouabain. 5. A transcutaneous active Na transport mechanism thus appears to be incipient in the mudpuppy but is limited by a low permeability of the outer barrier of the cells. 6. Vasotocin increased the skin's diffusion permeability for water but had no effect on the influx of Na or urea. 7. The function of Necturus skin is in several respects unique compared to that of other amphibians.