Acyclovir (ACV) is a widely used antiviral agent for the treatment of herpes simplex virus encephalitis. Dexamethasone (DEX) is often co-administered as an adjunctive therapy. Tariquidar (TQD), a potent and selective inhibitor of P-glycoprotein, has been demonstrated to enhance the brain accumulation of ACV. This study is designed to systematically investigate the impact of varying DEX dosages on the blood-brain barrier (BBB) transport of ACV, elucidate the mechanisms involved, and evaluate the therapeutic potential of TQD in the management of herpes simplex virus encephalitis. Microdialysis, coupled with high-performance liquid chromatography, was employed to measure unbound ACV concentrations in blood and brain tissue at pre-determined time intervals. The effects of low (0.5 mg/kg), moderate (1 mg/kg), and high (3 mg/kg) doses of DEX, administered continuously for 4 days, on the pharmacokinetic parameters of ACV were evaluated in the presence and absence of tariquidar (TQD, P-glycoprotein inhibitor, 8 mg/kg). The BBB transport of ACV was assessed by calculating the ratio of the area under the unbound brain drug concentration-time curve to the area under the unbound blood drug concentration over time, to reflect changes in the BBB transport (Kp,uu,brain) of ACV. The mean of the area under the unbound brain and blood drug concentration-time curve, and Kp,uu,brain in rats of the solvent control (SC) group were 2674 ± 448 min·μg/mL, 564 ± 136 min·μg/mL, and 20.9%, respectively. In the low-dose DEX group, the Kp,uu,brain of ACV was 19.5%, with no statistically significant difference compared with the SC group (P > 0.05). In the moderate- and high-dose DEX groups, the Kp,uu,brain of ACV decreased to 17.0% and 14.0%, respectively, with a statistically significant difference compared with the SC group (P < 0.001). Pairwise comparisons further revealed significant differences in the Kp,uu,brain of ACV among different DEX dose groups. Conversely, TQD administration increased the Kp,uu,brain of ACV by 1.40-fold (P < 0.001). The Kp,uu,brain in the TQD plus low-dose DEX group was 27.1%, significantly higher than that of the SC group (P < 0.001). The Kp,uu,brain of the TQD plus moderate-dose DEX and TQD plus high-dose DEX groups was 21.6% and 18.4%, respectively, with no significant difference compared to the SC group (P > 0.05). Low-dose DEX does not significantly alter the BBB transport of ACV. However, moderate to high doses of DEX exhibit a dose-dependent reduction in ACV penetration across the BBB. Co-administration of TQD with DEX counteracts this effect, and even enhancing the BBB transport of ACV.