The pharmacokinetics of a single intravenous dose of diazepam 0.1 mg/kg was studied in 6 healthy volunteers, in random order under controlled conditions and following pretreatment with its major metabolite, desmethyldiazepam (20 mg/kg/day) for one week. In the two subjects with the highest plasma concentration of desmethyldiazepam (990 and 1100 ng/ml, respectively), total plasma clearance (Cl) of diazepam was reduced after desmethyldiazepam, by 31% and 54%, respectively. In three individuals there was a moderate decrease of 14% to 21%, and no effect was seen in one volunteer. Cl was significantly reduced (11.5 +/- 1.8 vs. 9.1 +/- 3.3 ml/min; p = 0.015) and elimination half-life tended to be prolonged (38.5 +/- 10.4 vs. 65.8 +/- 67.1 h; p = 0.15). It is concluded that high concentrations of desmethyldiazepam can influence the elimination of its parent drug diazepam by product inhibition.