The effect of hypophy-sectomy on renal Na loss, Na uptake, and Na outflux of F. kansae was estimated from fish held in freshwater, during adjustment to sea water (1000 mos moles/kg), and after an 8-day exposure to sea water. The hypophysectomized animals showed a higher renal Na loss than did their controls (0.53 vs 0.27 [mu]eg Na+/g/hr.) in fresh water, and a slightly higher rate of Na uptake, 1.25 vs 1.10 [mu]eg/g/hr. Sodium outflux was identical for the 2 groups. Shortly after transfer into sea water, renal Na loss fell to a low value, 0.04 [mu]eg/g/hr. in both groups. By 20 hr. post-transfer, the control group renal Na loss was 0.48 [mu]eg/g/hr.; the hypophysectomized animals did not Increase renal Na loss. The 2 groups were identical after 8 days in sea water and renal Na loss was low, 0.05 [mu]eg/g/hr. Both groups were able to increase extra-renal Na loss after transfer into sea water, however, the ability to excrete salt was reduced in the hypophysectomized animals.