We investigated the effects of metabolic acidosis and compensated metabolic acidosis on force of contraction of the diaphragm in anesthetized dogs. Mechanically ventilated animals were prepared with an open thorax. A balloon was positioned beneath the diaphragm to measure transdiaphragmatic pressure (Pdi), and a plaster cast was placed around the abdomen to maintain length and geometry of the diaphragm. The force of contraction was evaluated by measuring Pdi during supramaximal phrenic stimulation at different frequencies and also during spontaneous inspiratory efforts. In 13 dogs with an arterial pH (pHa) of 7.38 and arterial PCO2 (PaCO2) of 36.5 Torr, metabolic acidosis was produced by infusion of HCl until pHa equaled 6.98 and PaCO2 equaled 36.4 Torr. Pdi at all frequencies greater than 10 Hz was significantly reduced (P less than 0.05). The dogs were then hyperventilated until pHa was 7.34 and PaCO2 was 12.8 Torr. Pdi was significantly reduced again at all frequencies (P less than 0.05) except 5 Hz. The percent reduction in Pdi by compensated acidosis was significantly greater at low-frequency stimulation than at high (P less than 0.05). Similar qualitative results were observed during spontaneous inspiratory efforts where Pdi was compared at constant magnitudes of diaphragmatic electromyograms. Twitch characteristics revealed that metabolic acidosis led to a significant shortening of twitch relaxation time (P less than 0.05), and compensated metabolic acidosis added to this effect a significant decrease in twitch amplitude (P less than 0.05).