1. Larval salamanders in pond water actively transport both Na+ and Cl- inwards. The two fluxes can occur independently, indicating that they are not linked obligatorily through a single mechanism. Ammonia is excreted extrarenally at rates comparable with active Na+ influx.

2. A potential difference exists across the body surface of larval salamanders under these conditions. Over the range 0.1-10 mM the transepithelial potential (TEP) varies approximately logarithmically with [NaCl]. Only Na+ and Li+ salts generate such a TEP, while K+, Rb+, Ca2+, Mg2+, and NH4+ salts have little effect. Azide and dinitrophenol abolish the TEP, while strophanthin has no effect. When active Na+ uptake is enhanced by salt-depletion, higher TEPs are generated. Conversely, animals with Na+ transport depressed by salt-loading show significantly smaller TEPs.

3. The magnitude of the TEP is uneffected by replacing NaCl with Na2SO4. Permeability of the body surface to SO42- is very low, and the similarity in electrical behaviour of the two anions suggests that Cl- penetration by diffusion is also small. This surmise is supported by two observations: (1) KCl generates no TEP over a wide range of concentrations; and (2) cupric ion has no effect on the TEP developed in dilute NaCl solution.

4. It is suggested that the TEP in salamanders is generated by active inward transport of Na+, but that in dilute solutions electrostatic neutrality is maintained by a cation-exchange system involving ammonium ion. Uptake of Cl- under these conditions appears to occur by an independent mechanism and is non-electrogenic.

This work was supported by grants G-12471 from the National Science Foundation and G-04254 from the National Institutes of General Medical Sciences.