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 NH4Cl. Analyses were made in the solution and in the muscles for K and NH3, and the muscles were also analyzed for Cl, HCO3, 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 NH3 which penetrated. An increased loss of K due to the penetration of NH3 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 NH3 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 NH3 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 NH4Cl into anesthetized cats causes an increase in the level of K in the blood plasma.