To determine the effect of fluoxetine on diazepam's pharmacokinetic and psychomotor responses, single oral doses of 10 mg diazepam were administered to six normal subjects on three occasions, either alone or in combination with 60 mg fluoxetine. Diazepam was given alone, after a single dose of fluoxetine, and after eight daily doses of fluoxetine. Psychometric data showed that fluoxetine had no significant effect on the psychomotor responses to diazepam. However, the pharmacokinetic data indicated a change in diazepam disposition after fluoxetine administration. Diazepam AUC was larger, the half-life was longer, and the plasma clearance was lower after fluoxetine administration, suggesting that fluoxetine inhibited the metabolism of diazepam. The reduced formation of an active metabolite, N-desmethyldiazepam, also suggested that fluoxetine inhibited diazepam's metabolism. The clinical implications of this pharmacokinetic drug-drug interaction are minor because psychomotor responses were unaffected and offsetting changes in the kinetics of diazepam and its metabolite occurred. Dosage modification of either fluoxetine or diazepam is unlikely to be necessary.