The utility of an osmotic rectal drug delivery system as a tool in steady-state pharmacokinetic interaction studies has been investigated using the cimetidine-antipyrine interaction. Antipyrine was administered to six healthy male volunteers at the rate of 15 mg/h until steady-state was reached. Cimetidine 400 mg was then given followed by 200 mg cimetidine after 2, 4 and 6 h. Antipyrine kinetics in plasma and saliva were assessed, and metabolite excretion was determined in urine. Antipyrine levels in plasma and saliva increased shortly after cimetidine administration, indicating inhibition of antipyrine metabolizing enzymes. From the metabolite data it was concluded that all major metabolic pathways of antipyrine were affected to the same extent. The effect lasted somewhat longer than anticipated on the basis of the plasma cimetidine concentrations, but it had disappeared within 48 hours after cessation of treatment. It is concluded that the osmotic rectal drug delivery system is a useful tool in pharmacokinetic interaction studies, because it provides very constant steady-state concentrations, thus permitting investigation of the time course of drug interactions.