Nefazodone is a potent and selective inhibitor of cytochrome P450 3A4 (CYP3A4), an enzyme pathway responsible for the biotransformation of a number of steroid compounds. The potential therefore exists that nefazodone inhibits the disposition of methylprednisolone. In this open label, repeated measures study, the effect of 9 days of nefazodone administration on the pharmacokinetic disposition of a single 0.6 mg/kg intravenous dose of methylprednisolone was assessed. Additionally the effect of concomitant nefazodone use on duration of cortisol suppression after methylprednisolone administration was assessed. Eight healthy volunteers completed the study. Following nefazodone administration, the mean (+/-SD) area under the methylprednisolone concentration-time curve was significantly higher (1393 +/- 343 vs. 2966 +/- 928 ug*h/L; P < 0.005), apparent clearance was lower (28.7 +/- 7.2 vs. 14.6 +/- 7.8 L/h; P < 0.02) and the terminal elimination half-life was longer (2.28 +/- 0.49 vs. 3.32 +/- 0.95 hours; P < 0.02). The duration of cortisol suppression after methylprednisolone administration was longer (> or =32 vs. 23.3 +/- 3.43 hours) during nefazodone administration.