This study was performed to determine the effect of omeprazole, given in therapeutically recommended doses, on the steady-state plasma levels of phenytoin in epileptic patients. Five men and three women of median age 34 years participated in the study. Steady-state plasma levels of phenytoin were measured once a week for 2 weeks before and after, respectively, and during 3 weeks of concomitant omeprazole treatment with 20 mg daily. Urinary excretion of phenytoin and its metabolite [5-(p-hydroxyphenyl)-5-phenyl-hydantoin] were determined before and at the end of the omeprazole treatment period. The steady-state plasma phenytoin levels as well as urinary excretion of phenytoin and its main metabolite were unchanged during omeprazole treatment. The results from this study suggest that concomitant omeprazole treatment in therapeutically recommended doses (20 mg daily) will not significantly affect the steady-state plasma levels of phenytoin in epileptic patients.