ACT-539313 is a potent and selective orexin-1 receptor antagonist. CYP3A is the major cytochrome P450 (CYP) enzyme involved in the metabolism and clearance of ACT-539313 in man. The main objective of this study was to investigate the effect of ACT-539313 on the pharmacokinetics of orally administered midazolam. Thereby, this single-center, open-label, fixed-sequence study investigated the CYP3A interaction potential of ACT-539313 following single- (on day 2) and repeated-dose (on day 11) twice-daily administration of 200 mg ACT-539313. Exposure to midazolam was higher during concomitant administration of single as well as after repeated doses of ACT-539313 over 10 days compared to midazolam alone (day 1). In the presence of ACT-539313, the geometric mean ratio of the maximum plasma concentration and the area under the plasma concentration-time curve from time 0 to 24 hours increased by 1.18- and 1.79-fold on day 2, and by 2.13- and 4.54-fold on day 11, respectively. A similar outcome was also shown in the additionally evaluated urinary 6β-hydroxycortisol/cortisol ratio (6β-CR), as the geometric mean ratio of the 6β-CR showed a decrease to 0.78 on day 2 and to 0.61 on day 11. The most commonly reported adverse events (AEs) included somnolence and headache. All AEs were transient and of mild intensity. No treatment-related effects on vital signs, clinical laboratory, and electrocardiogram were observed. In summary, the observed corresponding decrease of both the validated, exogenous (midazolam/1-hydroxymidazolam ratio) and a frequently used endogenous (6β-CR) marker of CYP3A activity is indicative of CYP3A inhibition occurring after ACT-539313 treatment.