The objective of the present investigation was to characterize the pharmacodynamic interaction between the synthetic neuroactive steroid alphaxalone and the benzodiazepine midazolam. The time course of the electroencephalographic (EEG) effect (11.5-30 Hz) was determined in rats in conjunction with plasma concentrations. Alphaxalone was administered as a continuous intravenous infusion of 0, 1.2, 2.2, or 5.2 mg over 360 min. Midazolam was administered as a 5-min intravenous bolus infusion of 4 mg.kg-1. The pharmacokinetic profiles of both drugs were described by a two-compartment model. No pharmacokinetic interaction was observed. The EEG effect versus time profiles of midazolam and alphaxalone, when administered separately and in combination, were modeled on the basis of the recently proposed mechanism-based pharmacokinetic/pharmacodynamic model for GABAA receptor modulators, which contains separate expressions to describe the drug-receptor interaction and the stimulus-response relationship. The pharmacodynamic interaction between alphaxalone and midazolam was best characterized using an independent drug-drug interaction model without an expression for allosteric modulation of the effect of midazolam by alphaxalone. The final model contained an exponential expression to account for acute functional adaptation to the EEG effect upon continuous infusion of alphaxalone. The mechanism-based analysis showed that this functional adaptation is best explained by a change in the system-specific stimulus-response relationship, rather than the drug-receptor activation process. It is concluded that the pharmacodynamic interaction between alphaxalone and midazolam in vivo is best described using an independent interaction model without allosteric modulation.