In eight normal volunteers, the combination of ethanol (0.5 g/kg) and diazepam (10 mg) administered orally produced a greater decrease in motor performance on a pursuit rotor than diazepam alone. The pharmacologic effect of diazepam was enhanced by 73% and this potentiation was associated with significantly greater diazepam concentrations (p less than 0.01) than after diazepam alone. The failure to observe any increase in the concentrations of the principal metabolite, N-desmethyl diazepam, during the period of enhanced pharmacologic effect precludes any change in the demethylating metabolic process as being responsible. The data suggest (0.10 greater than p greater than 0.05) a trend to a smaller volume of distribution of diazepam when ethanol is administered prior to diazepam ingestion. The subjects showed acute tolerance to the effects of diazepam. Lower plasma concentrations on the ascending side of the plasma diazepam concentration versus time profile were linked with the same pharmacologic responses associated with a greater drug concentration on the descending portion, of the same curve.