The disposition and metabolism of [3H]pioglitazone was determined in male rats after oral administration. The peak plasma concentration of 10 micrograms/ml occurred 1 hr after dosing at 10 mg/kg p.o.; the apparent plasma terminal half-life was 7.5 hr. Most of the radioactivity in plasma up to 8 hr after dosing was due to the parent drug. Pioglitazone was highly protein-bound in plasma; only 1-2% was free at concentrations of 0.1-10 micrograms/ml. Within 3 days after oral administration to bile duct-cannulated rats, 36% and 15% of the oral dose was recovered in the bile and urine, respectively. The pattern of biliary and urinary metabolites was similar. A total of eight metabolites were isolated and identified on the basis of NMR spectroscopy and MS. Metabolites resulting from hydroxylation of either carbon adjacent to the pyridine ring were conjugated with glucuronic acid (M7) or sulfuric acid (M6). The metabolite hydroxylated on the terminal carbon of the ethyl side chain was further oxidized to the carboxylic acid derivative (M3). Oxidative loss of the terminal carbon led to a nicotinic acid derivative (M2) and loss of both carbon atoms to the corresponding 3-hydroxypyridine (M9) derivative that was excreted as the sulfate conjugate (M8). The two carboxylic acid metabolites were also conjugated with taurine (M4 and M5).