To clarify the relationship between intracellular concentrations of methylmalonic acid and metabolic and growth inhibition in vitamin B12-deficient rats, hepatic methylmalonic acidlevels were assayed and inhibition of glucose and glutamic acid metabolism by methylmalonic acid was studied in isolated hepatocytes. Vitamin B12-deficient rats (14 weeks old) excreted more urinary methylmalonic acid and had lower body weights than the control rats. Hepatic methylmalonic acid levels (3·6 (SD 1·30)–5·3 (SD 0·51) µmol/g tissue; 7·9 (SD 2·90)–11·8 (SD 1·14) mM) were increased and correlated with the extent of the growth retardation during vitamin B12-deficiency. Isolated hepatocytes and mitochondria from normally fed rats were labelled with [14C(U)]glucose and [14C(U)]glutamic acid respectively, in the presence or absence of 5mM-methylmalonic acid. Although methylmalonic acid did not affect the incorporation of14C into protein and organic acid fractions in the hepatocytes, it inhibited14CO2formation (an index of glucose oxidation by the Krebs cycle) by 25% and incorporation of14C into the amino acid fractionby 30%. In the mitochondria, methylmalonic acid inhibited14CO2, formation (indicating glutamic acid oxidation by the Krebs cycle) by 70%, but not the incorporation of14C into the protein fraction. The incorporation of14C into the organic acid fraction was significantly stimulated by the addition of methylmalonic acid. These results indicate that the unusual accumulation of methylmalonic acid caused by vitamin B12-deficiency disrupts normal glucose and glutamic acid metabolism in rat liver, probably by inhibiting the Krebs cycle.