Urinary excretion of valproate (VPA, administered as its sodium salt) and its various metabolites was studied in rats before and after 6 weeks of chronic treatment with intraperitoneal (i.p.) injection of 200 mg/kg VPA three times daily. Urinary excretion was determined after i.p. injection of a single dose of 200 mg/kg VPA, to which [14C]labeled VPA had been added to yield a dose of 1 microCi/kg body weight. Unlabeled VPA and metabolites were determined in urine by gas chromatography-mass spectrometry (GC-MS). After injection of a single dose of VPA before onset of chronic treatment, approximately 40-50% of the dose administered was excreted in the urine within 24 h, mainly in the form of conjugated VPA and omega-oxidation products, i.e., 5-hydroxy-VPA and 2-propyl-glutaric acid. After chronic treatment, urinary excretion of total radioactivity increased approximately 75% as compared with activity before chronic treatment, demonstrating a marked increase in elimination rate of VPA during prolonged administration. Determination of VPA and metabolites in urine by GC-MS indicated that this enhanced elimination resulted mainly from increases in glucuronidation and beta-oxidation of VPA, whereas omega-oxidation was apparently not altered or was even reduced. The data strongly indicate that at least in rats VPA produces induction of its own metabolism during prolonged treatment.