It is assumed that the development of metabolic acidosis during sepsis is secondary to lactic acidosis. We assessed the composition of the anion gap during severe sepsis induced by cecal perforation in rats. In the first experiment, cardiac output, arterial blood gases, and arterial lactate were measured over a 6 hr interval in five septic rats and in five rats serving as sham-operated controls. The cardiac output decreased from 331 +/- 32 to 172 +/- 9 ml/kg/min (P less than 0.01) in the septic rats. Although the arterial lactate was increased to 2.1 +/- 0.2 mEq/L in septic rats compared to 0.8 +/- 0.1 mEq/L in sham rats (P less than 0.01), the HCO3- was decreased to 16.5 +/- 0.6 mEq/L in septic rats versus 23.8 +/- 1.10 mEq/L in sham rats (P less than 0.01). We further investigated this bicarbonate deficit in a second study in which arterial blood was sampled at 6 hr for blood gases, and plasma Na+, K+, Cl-, HCO3-, lactate, pyruvate, beta-hydroxybutyrate, acetoacetate, citrate, creatinine, albumin, and amino acids in five septic and five sham rats. The serum anion gap was calculated as [(Na(+) + K+) - (Cl(-) + HCO3-)]. The anion gap was 21.6 +/- 1.6 mEq/L in the septic animals as compared to 13.2 +/- 0.5 mEq/L in the sham animals (P less than 0.01). There were no differences in the concentration of pyruvate, beta-hydroxybutyrate, acetoacetate, citrate, creatinine, albumin, or amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)