The effects of steady state dosing with omeprazole and cimetidine on plasma diazepam levels have been studied in 12 healthy males. Single doses of diazepam (0.1 mg.kg-1 i.v.) were administered after one week of treatment with omeprazole 20 mg once daily, cimetidine 400 mg b.d. or placebo, and the treatment was continued for a further 5 days. Blood was collected for 120 h after the dose of diazepam for the measurement of diazepam and its major metabolite desmethyl diazepam. The mean clearance of diazepam was decreased by 27% and 38% and its half-life was increased by 36% and 39% after omeprazole and cimetidine, respectively. Neither drug had any apparent effect on the volume of distribution of diazepam. Desmethyldiazepam appeared more slowly after both omeprazole and cimetidine. It is concluded that the decrease in diazepam clearance was associated with inhibition of hepatic metabolism both by omeprazole and cimetidine. However, since diazepam has a wide therapeutic range, it is unlikely that concomitant treatment with therapeutically recommended doses of either omeprazole or cimetidine will result in a clinically significant interaction with diazepam.