The relative effects of respiratory and metabolic acidosis on diaphragm function are not known. To determine these effects, we compared the effects of respiratory and lactic acidosis on the contractile properties of the diaphragm. We estimated diaphragmatic performance from the change in transdiaphragmatic pressure after supramaximal stimulation of the phrenic nerves in an open-chested, casted-abdomen dog. Similarly, we stimulated the gastrocnemius motor nerve and examined force production and relaxation rate to determine if there was a difference in the response of this skeletal muscle. There was a fall in diaphragm performance with respiratory acidosis (77.1 +/- 16.9 cm H2O versus 93.8 +/- 15.0 cm H2O baseline), but not with lactic acidosis (96.7 +/- 15.7 cm H2O versus 93.8 +/- 15.0 cm H2O baseline); and the gastrocnemius was unaffected by either acidosis. The changes with respiratory acidosis were similar to those seen with diaphragmatic fatigue and had similar relaxation rate changes, suggesting that intracellular pH may play a mechanistic role in respiratory muscle fatigue. In addition, the absence of a respiratory acidosis effect on a non-diaphragmatic skeletal muscle's function represents another physiologic difference between the diaphragm and other skeletal muscles.