The metabolism of aminoglutethimide was studied in the rat by use of the 14C-labeled compound. Following oral doses of 5 and 50 mg/kg, the drug was almost completely eliminated within 48 hr into urine and feces, mostly in the form of metabolites. In bile duct-cannulated rats, biliary excretion of radioactivity amounted to about 52% within 24 hr of an orally administered 50 mg/kg dose, with the remainder of the dose being eliminated into urine. The major urinary metabolites resulted from acetylation of the aniline moiety, hydroxylation of the glutarimide ring at positions 3 and 4, and oxidative elimination of the ethyl sidechain. The polar metabolites are accounted for by aromatic hydroxylation with subsequent sulfate conjugation and by a glutarimide ring-opened compound. In addition, a gamma-butyrolactone derivative was also identified.