Abstract 1. 1. Frogs were acclimated to total immersion for 7–14 days in NaCl solutions in water from Lago Maggiore, over the range of tolerance from pure lake water, 0.1 mM·.l −1, to 150 mM. 2. 2. Fluxes of Na + were calculated from continuous records of radioactivity of the medium, after injection of 22NaCl, for periods of 18–20 hr. Calculations were based on initial and final values of concentration of total and isotopic Na + in blood plasma and medium. 3. 3. Algorithms for flux calculation were based on analytic solution of flux equations for a 4-compartment closed system. 4. 4. Plasma [Na +] and extracellular volume (Na space) were regulated near 112 mM and 6 ml respectively. Plasma [Na +] was greater than medium [Na +] in all concentrations from 0.1 to 100 mM: in 150 mM, there was no significant difference between plasma and medium [Na +]. 5. 5. Unidirectional flux rates increased with increasing concentration of medium, but rates of influx from medium were not statistically different from rates of outflux to the medium in any concentration. 6. 6. In controls, influx rate to the plasma was less than influx rate from the medium, indicating the presence of an indirect channel of influx (gut) in addition to direct influx across the skin. 7. 7. Outflux rates from plasma were less than those to the medium, indicating the presence of an indirect channel of outflux (urinary) in addition to leakage across the skin. 8. 8. Evidence of urinary elimination of isotopic Na + was obtained for all concentrations of medium. 9. 9. Flux rates in the most concentrated media were increased by removal of the anterior lobe of the pituitary, as was outflux from plasma in lake water. These effects were not associated with any modification of regulatory ability.