This study was conducted to determine the potential for an interaction between nefazodone, a new antidepressant, and triazolam after a single dose of triazolam and multiple doses of nefazodone in a randomized, double-blind, placebo-controlled study in healthy male volunteers. The metabolism of triazolam is dependent on cytochrome P450 3A4, and because nefazodone has been shown in vitro to be an inhibitor of this isoenzyme, this study was conducted to assess the potential for an interaction between the two drugs. Twelve subjects were assigned to one of two groups and received an oral dose of either placebo or 0.25 mg of triazolam on days 1 and 2. Nefazodone (200 mg) was administered twice daily from the evening of day 2 to the morning of day 9. Subjects received either 0.25 mg of triazolam or placebo with the nefazodone dose on the mornings of days 8 and 9. Serial blood samples were collected on the mornings of days 1, 2, 8, and 9 for the analysis of triazolam by a validated gas chromatography/electron capture detection method and on days 8 and 9 for the analysis of nefazodone and its metabolites, hydroxynefazodone (HO-nefazodone) and m-chlorophenylpiperazine (mCPP), by a validated high-performance liquid chromatography/ultraviolet method. Noncompartmental pharmacokinetic analysis showed that there was no effect of triazolam on the pharmacokinetics of nefazodone, HO-nefazodone, or mCPP after the coadministration of triazolam and nefazodone. There was a significant effect of 200 mg of nefazodone twice daily on the pharmacokinetics of triazolam. Mean triazolam peak concentration values increased (p = 0.003) from 2.33 to 3.88 ng/ml when triazolam was administered alone and in combination with nefazodone, respectively. Corresponding mean triazolam area under the curve values increased (p < 0.001) from 8.14 to 31.74 ng.h/ml. The plasma protein binding of triazolam was approximately 85% when triazolam was given alone and when given concurrently with nefazodone. The increase in triazolam concentrations in plasma appears to be attributable to the inhibition of cytochrome P450 3A4 metabolism by nefazodone. If triazolam is coadministered with nefazodone, a reduction in the triazolam dosage is recommended; no dosage adjustment is required for nefazodone.