[l-(14)C]Propionate administered intravenously was metabolized to methylmalonate, to 3-hydroxypropionate, and to methylcitrate in the urine of a patient with methylmalonic acidemia. L-[U-(14)C]Isoleucine and L-[U-(14)C]valine were also converted to urinary methylmalonate and to 3-hydroxypropionate in the patient. Two patients with propionic acidemia due to a defect in propionyl-CoA carboxylase metabolized [l-(14)C]propionate to uninary methylcitrate and 3-hydroxypropionate. The appearance of radioactive 3-hydroxypropionate in the urine after the administration of these compounds indicates that beta-oxidation of propionyl-CoA through acryloyl-CoA was functioning in these patients. The conversion of valine to 3-hydroxypropionate suggests that valine is oxidized by way of propionate and propionyl-CoA in man.