Both Ca and Mg in isolated frog skeletal muscle can contribute to the buffering of pH. In low pH solutions, there is a decrease in both muscle Ca and Mg, and in high pH solutions, there is an increase in both muscle Ca and Mg. The muscle content of these ions as compared to control values in muscles immersed in solutions of pH 2.2 was 25.7 [plus or minus] 0.67% for Mg but only 63.9 [plus or minus] 1.46% for Ca. The muscle content of these ions as compared to control values in muscles immersed in solutions of pH 10.0 was 247.5 [plus or minus] 2.96% for Ca but only 123.3 [plus or minus] 0.60% for Mg.