To assess possible pharmacokinetic and pharmacodynamic interactions between the antidepressant venlafaxine and diazepam, a randomized, two-period, crossover study was conducted in 18 men. Multiple-dose venlafaxine (50 mg every 8 hours) or placebo (double-blind) was given for 10 days; on day 4 a single placebo dose (same appearance as diazepam capsule, single-blind) was given; and on day 5 a single dose of diazepam (10 mg) was given. Pharmacokinetic data indicated that diazepam had no significant effect on venlafaxine or O-desmethylvenlafaxine disposition. Diazepam pharmacokinetics were minimally changed in the presence of venlafaxine. Diazepam oral clearance (CL/f) increased slightly (24 +/- 8 versus 26 +/- 6 mL/h/kg; P = .007), volume of distribution (Vz/f) increased (0.85 +/- 0.28 versus 0.99 +/- 0.34 L/kg; P = .02), and AUC decreased (5973 +/- 2304 versus 5008 +/- 1354 ng.h/mL; P = .02). Venlafaxine did not alter desmethyldiazepam pharmacokinetics. Pharmacodynamic data showed a statistically significant diazepam-venlafaxine interaction for only one of the eight psychometric tests given. Critical flicker fusion slightly decreased (P = .01) between placebo-diazepam (37.85 +/- 3.28 Hz) and venlafaxine-diazepam (37.09 +/- 4.13 Hz) treatments. The observed pharmacokinetic and pharmacodynamic interactions between diazepam and venlafaxine were small and probably clinically insignificant.