In experiments on the frog Rana ridibunda and albino rats, studies have been made on water and electrolyte excretion after intravenous injection of isotonic solution of NaCl, water and saline (0.35% solution of NaCl for frogs and 0.45% solution for rats) loadings which amounted to 3% of the body weight. All types of this stimulation increase renal excretion of sodium and potassium in rats. In frogs, intravenous injections of isotonic solution of NaCl increased sodium and potassium excretion to a lower extent, as compared to that in rats; in experiments with water and saline loadings, the excretion of sodium increased to a higher extent, whereas potassium excretion remained practically unchanged. The increase of water and electrolyte excretion on frogs is due to the appearance in the blood of a humoral natriuretic factor, which in these animals does not practically affect potassium excretion.