The relationships between oxygen consumption (Q(O2)) and calculated cytoplasmic ADP concentration ([ADP]) and the free energy of ATP hydrolysis (deltaG(ATP)) were examined in ex vivo arterially perfused cat soleus muscles during repetitive twitch stimulation under normocapnic (5% CO2) and hypercapnic (70% CO2) conditions. Hypercapnia decreased extra- and intracellular pH by over 0.5 but had no significant effect on Q(O2) or phosphocreatine (PCr)/ATP in muscles at rest. The maximum Q(O2) measured during stimulation and the rate constant for PCr recovery after stimulation both decreased during hypercapnic compared with normocapnic perfusion, but the estimated ATP/O2 was unchanged. The change in PCr and deltaG(ATP) with increasing Q(O2) was greater during hypercapnic compared with normocapnic stimulation, as expected from the decrease in maximum Q(O2). However, the relationships between Q(O2) and [ADP] and deltaG(ATP) were both shifted to the left during hypercapnia compared with normocapnia. The results show that changes in cytoplasmic adenine nucleotides and phosphate are not sufficient to explain the control of respiration in skeletal muscle. However, in the context of thermodynamic models of respiratory control, the results can be explained by increased intramitochondrial potential for ATP synthesis at low pH.