SUMMARY

The hypophysectomized eel (Anguilla anguilla L.) in fresh water showed a marked decline in serum Na and Ca concentrations whilst serum K con-centration was elevated compared with sham-operated controls. Muscle was hyperhydrated with decline in Na, K and Mg concentrations. The total amount of Na and K/kg. fat-free dried muscle was decreased. The ratio serum [Na]: [K] was lowered. Injection of mammalian prolactin rendered water content normal but not the electrolyte values. Injections of cortico-trophin or cortisol brought electrolyte levels towards normal values though muscle hyperhydration was still evident. Injection of prolactin (2 mg./100g. body weight/day) plus cortisol (20 \g=m\g./100g. body weight/day) rendered the values for water and electrolyte composition similar to those for intact and sham-operated controls. The results are discussed in terms of the equivalence of mammalian prolactin to the fish hormone, paralactin, and of cortisol as a prime agent in controlling electrolyte balance in the Teleostei