1. A study was made of the electrolyte changes which occur when frog muscles are immersed in a Ringer solution with 1/5 of the Na replaced by NH 4 Cl. Analyses were made in the solution and in the muscles for K and NH 3 , and the muscles were also analyzed for Cl, HCO 3 , and Na. Control muscles were immersed in normal Ringer's solution and similarly analyzed. 2. The amount of ammonia taken up was about equal to the K and Na lost. There was also a small increase in chloride content. The bicarbonate content was the same in both experimental and control muscles, indicating no change in the muscle pH due to the NH 3 which penetrated. An increased loss of K due to the penetration of NH 3 was also demonstrated by the use of radioactive K. 3. After 5 hours, the concentration of ammonia per gram of muscle is about the same as the concentration in the solution. After 4 or 5 days, the concentration in the muscle is about 1.5 times that in the solution. The inside to outside NH 3 ratio is about equal to the corresponding H ion ratio, but is much less than the K ratio. 4. The rate of penetration of the NH 3 is increased by a rise of temperature, by stirring the solution, and by decrease in the concentration of Na, K, Ca, or Mg in the solution; it is decreased by increasing the size of the muscles or by killing them with chloroform or boiling. 5. Liver, smooth muscle, skin, and kidney, in a few experiments, behaved much like muscle except that there was a formation of urea in the case of liver. 6. The injection of NH 4 Cl into anesthetized cats causes an increase in the level of K in the blood plasma.