In the hypophysectomized euryhaline flounder Platichthys flesus (L.) the ionic composition of the blood is almost normal in seawater (SW), but sodium and chloride levels are reduced in freshwater (FW), owing to increased extrarenal losses. Radioisotopic methods have been used to study the kinetics of sodium ( 24Na) and water (tritiated water, HTO) balance in SW, 1 3 SW and FW. In SW sodium turnover, 45.1% exchangeable Na hr , and total ion efflux, 2760 μeq/hr/100g, of intact fish were reduced by one-fifth in hypophysectomized fish. It is probable that both the Na:Na and Na:K (Na pump) components of branchial exchange were depressed; maintenance of a normal blood sodium and renal ion efflux was indicative of a parallel reduction in intestinal ion uptake. An ACTH-cortisol deficiency may have been responsible for these effects. In 1 3 SW sodium turnover, 3.3%/hr, was similar in intact and ablated fish. The efficiency of the instantaneous regulation of sodium efflux on abrupt SW → FW transfer was reduced by hypophysectomy, leading to an increase in the residual passive efflux. During FW-adaptation following 1 3 SW → FW transfer branchial efflux attained stable levels in intact and ablated fish but was higher (+100% after 5 days) in the latter group. Influx was similar in the two groups, and, expressed both as an absolute flux and as f in Na ext increased progressively. Intact fish achieved positive sodium balance, but ablated fish remained in negative balance due to the persistent high efflux. Though prolactin treatment did not repair this defect in long-term hypophysectomized fish, it did halve sodium turnover in intact, SW-adapted fish, an effect interpreted as a reduction in passive branchial sodium permeability. In intact fish in SW, HTO turnover (15.4%/hr) remained similar in 1 3 SW, and in FW (<1 week), but later increased (>1 week, 23.1% hr). In ablated fish the turnover values were similar and were not altered by prolactin treatment. Impaired sodium conservation in hypophysectomized fish in FW is due solely to a high branchial efflux; the sodium absorption pump and aqueous permeability are unaffected. These results are discussed in relation to studies on other hypophysectomized teleosts.