The structures of seven urinary metabolites of omeprazole following high oral doses to rats and dogs were determined unambiguously by combining different analytical and spectroscopic techniques including derivatization and stable isotopes. Omeprazole was metabolized by aromatic hydroxylation at position 6 in the benzimidazole ring followed by glucuronidation. There was also oxidative O-dealkylation of both methoxy groups, and aliphatic hydroxylation of a pyridine methyl group followed by oxidation to the corresponding carboxylic acid. Due to the experimental design, implying no pH control of collected samples, all metabolites were isolated as sulfides. They were formed in both species with quantitative variations in the metabolic pattern. As far as identified metabolites are concerned, aromatic hydroxylation and subsequent glucuronide formation were the major biotransformation routes in the dog. In the rat, aliphatic hydroxylation and the formation of the carboxylic acid represented the major metabolic pathways. The identified metabolites corresponded approximately to 50% (rat) and 70% (dog) of the amount excreted in the 0-24-hr urine (about 12% of the given dose in both species).