1. Regulation of glycogenolysis in frog skeletal muscle at rest and following contraction was studied by measuring the concentration of phosphate-containing metabolites and the intracellular pH (pHi) in CN-treated muscles, in which oxidative phosphorylation was inhibited by NaCN, using the 31P nuclear magnetic resonance (NMR) technique. 2. When CN-treated muscles were kept at rest, the phosphocreatine (PCr) concentration very slowly decreased with time with a corresponding increase of the inorganic phosphate (Pi) concentration, while the ATP concentration remained unchanged. The pHi changed in the alkaline direction for the first 3 h, and then started to change in the acidic direction. 3. When CN-treated muscles were tetanized for 10 s, the PCr concentration decreased with a corresponding increase of the Pi concentration and acidification of pHi, while the ATP concentration remained unchanged. 4. When CN-treated muscles were tetanized repeatedly (each for 2 s) at constant intervals, the pHi changed in the alkaline direction following the first and the second tetani, and then changed in the acidic direction following the subsequent tetani, indicating that the consumed ATP is first replenished by the Lohmann reaction, while glycogenolysis starts only when the total amount of contractile activity exceeds a critical value. 5. Irrespective of whether CN-treated muscles were kept at rest or tetanized repeatedly, the Pi concentration increased to about 8 mM (mmol/kg wet muscle) when glycogenolysis started, suggesting that the onset of glycogenolysis in CN-treated muscles is regulated by the Pi concentration. 6. The 'internal' buffering power of muscle cytosol was estimated to be 35 mM H+/pH unit in anaerobic muscles and 25 mM H+/pH unit in CN- and iodoacetic acid (IAA)-treated muscles. The 'internal' buffering power contains a contribution due to flux of carbon dioxide and lactic acid across the cell membrane. Evidence indicated that lactic acid flux is small.