During muscular fatigue two metabolites, hydrogen ions (H+) and inorganic phosphate (Pi), increase in concentration. The effect of increase in [H+] has been modeled mathematically for a system containing creatine kinase (EC 22.214.171.124), adenylate kinase (EC 126.96.36.199), and the appropriate concentrations of their substrates. Assuming that no other equilibrium reactions are involved, the result of acidification should be a useful increase in the ratio [ATP]/[ADP]. It is also shown by a reanalysis of earlier 31P NMR studies that the observed combination of increased [H+] and increased [Pi] leads to an increase in the monobasic phosphate concentration [Pi-] that is inversely proportional to the force of contraction. This suggests that Pi- may be a direct inhibitor of the actomyosin ATPase system.