1. The in vitro metabolism of omeprazole was studied in human liver microsomes in order to define the secondary metabolic pathways and identify the cytochrome P450 (CYP) isoforms responsible for the formation of the secondary metabolites of omeprazole. 2. The major secondary omeprazole metabolite was the hydroxysulphone, which was formed during incubation with both hydroxyomeprazole and omeprazole sulphone. A second metabolite, tentatively identified as pyridine-N-oxide omeprazole sulphone, was also formed during incubation with omeprazole sulphone. The formation kinetics of these two metabolites from omeprazole sulphone were biphasic suggesting the involvement of multiple CYP isoforms in each case. In contrast, the formation kinetics of hydroxysulphone from hydroxyomeprazole were linear. 3. Inhibition studies, performed with omeprazole sulphone as substrate at concentrations at which the high affinity activities predominated, with a series of isoform selective inhibitors as well as with an anti-CYP2C3 antibody suggested a dominant role of S-mephenytoin hydroxylase in the formation of hydroxysulphone from omeprazole sulphone. By contrast, CYP3A activities were predominant in the formation of hydroxysulphone from hydroxyomeprazole as well as in the formation of pyridine-N-oxide omeprazole sulphone from omeprazole sulphone.