This study examines the effects of genetically determined S-mephenytoin 4-hydroxylation capacity and cigarette smoking on the single-dose pharmacokinetics of oral alprazolam in 12 healthy male volunteers. Six subjects each were extensive metabolizers (EMs) and poor metabolizers (PMs) of S-mephenytoin 4-hydroxylation. Seven subjects were smokers (> 10 cigarettes/day), and five were nonsmokers, according to their self-reports. Each subject took a single oral dose of 0.8 mg of alprazolam, and blood samples were collected up to 48 hours postdose. Psychomotor function was assessed at times of blood samplings using the Digit Symbol Substitution Test (DSST), Visual Analog Scale (VAS), and UKU Side Effect Rating Scale. Plasma alprazolam concentrations were measured by a high-performance liquid chromatography assay. None of the mean pharmacokinetic parameters was significantly different between the EM and PM phenotype groups. Although the mean elimination half-life was significantly shorter in the smoker group (p < .01) than in the nonsmoker group (13.1 +/- 2.9 vs. 20.0 +/- 2.7 hours, mean +/- SD), other pharmacokinetic parameters did not differ significantly between the two groups. Psychomotor function parameters did not differ significantly either between the EM and PM groups or between the nonsmoker and smoker groups. The present study thus suggests that neither S-mephenytoin 4-hydroxylation status nor self-reports of extensive cigarette smoking has a major impact on the metabolism of alprazolam in humans.