Metabolism of propionate in human subjects was studied using bolus administration of 1-13C-propionate i.v. or orally. The study population consisted of five patients with propionic acidemia (PA), eight with methylmalonic acidemia (MMA; four responsive to vitamin B12), one each with multiple carboxylase deficiency and transcobalamin-II deficiency, and five healthy volunteers. Concentrations of 1-13C-propionate were measured in blood in three patients with PA, two with MMA, and two controls. Breath samples were obtained at intervals during 3 h after the dose, isotopic enrichment of 13CO2 was measured, and the cumulative percentage of recovery of 13C was calculated from the individual's predicted resting energy expenditure. Recovery of 13CO2 and half-time of 1-13C-propionate in PA were significantly less than normal. The same parameters in MMA were below normal, but significantly greater than in PA. Recovery of 13CO2 was well correlated with clinical severity in PA, but did not correlate in MMA. Differences between MMA and PA may indicate different distribution of propionate pools, differences in inducibility of residual enzyme activities, or an alternate pathway for decarboxylation of propionate available in MMA but not PA. Only one patient with PA demonstrated increased 13CO2 production during biotin treatment. In a B12-responsive MMA patient, no differences were noted within 2 d of initiating treatment with B12, but there was an increase in 13CO2 production after 4 mo. Recovery of 13CO2 was normal in the patient with transcobalamin-II deficiency before and after treatment with vitamin B12.(ABSTRACT TRUNCATED AT 250 WORDS)