We sought to understand the mechanism of metabolic acidosis that results in acute resuscitated endotoxic shock. In six pentobarbital-anesthetized dogs, shock was induced by Escherichia coli endotoxin infusion (1 mg/kg) and was treated with saline infusion to maintain mean arterial pressure > 80 mmHg. Blood gases and strong ions were measured during control conditions and at 15, 45, 90, and 180 min after endotoxin infusion. The mean saline requirement was 1833 ± 523 mL over a 3 h period. The total acid load from each source was calculated using the standard base deficit. The mean arterial pH decreased from 7.32 to 7.11 (p < .01); pco2 and lactate were unchanged. Saline accounted for 42% of the total acid load. However, 52% of the total acid load was unexplained. Although serum Na+ did not change, serum Cl- increased (127.7 ± 5.1 mmol/L vs. 137.0 ± 6.1 mmol/L; p = .016). We conclude that saline resuscitation alone accounts for more than one-third of the acidosis seen in this canine model of acute endotoxemia, whereas lactate accounts for less than 10%. A large amount of the acid load can be attributed to differential Na+ and Cl- shifts from extravascular to vascular spaces.