The purpose of this study was to investigate the effects of resveratrol, an antioxidant, on the pharmacokinetics of diltiazem and its active metabolite, desacetyldiltiazem, in rats. The pharmacokinetic parameters of diltiazem and desacetyldiltiazem were determined after an oral administration of diltiazem (15 mg/kg) to rats in the presence and absence of resveratrol (0.5, 2.5, and 10 mg/kg). Compared to the control group, the presence of resveratrol significantly (P < 0.05) increased the area under the plasma concentration-time curve (AUC) of diltiazem, except for resveratrol 0.5 mg/kg. Consequently, the absolute bioavailability (AB) of diltiazem in the presence of resveratrol (2.5 and 10 mg/kg) was significantly (P < 0.05) higher (10.2-11.1%) than that of the control (6.9%). The relative bioavailability (RB) of diltiazem in the presence of resveratrol (2.5 and 10 mg/kg) was increased by 1.48- to 1.60-fold. Resveratrol did not alter absorption rate constant (K(a)) and the time to reach the peak concentration (T(max)) of diltiazem. The AUC of desacetyldiltiazem was increased significantly (P < 0.05) in the presence of 10 mg/kg of resveratrol. The metabolite-parent AUC ratio (MR) in the presence of resveratrol was decreased but did not show significant change. In conclusion, resveratrol significantly increased the bioavailability of diltiazem due to the inhibition of both the cytochrome P450 (CYP) 3A4-mediated metabolism and the efflux pump P-glycoprotein (P-gp) in the intestine and/or liver. Based on these results, if these results would be confirmed in clinical experiments, the dosage of diltiazem should be readjusted when diltiazem is used concomitantly with resveratrol.