The purpose of this study was to investigate the possible effects of simvastatin, an HMG-CoA reductase inhibitor, on the pharmacokinetics of diltiazem and its main metabolite, desacetyldiltiazem, in rats. HMG-CoA reductase inhibitors and diltiazem could be prescribed for the prevention or treatment of cardiovascular diseases as a combination therapy. The pharmacokinetic parameters of diltiazem and desacetyldiltiazem were determined after oral administration of diltiazem (12 mg/kg) to rats with and without simvastatin (0.3 and 1.0 mg/kg). Compared to the control (diltiazem alone), simvastatin significantly altered the pharmacokinetic parameters of diltiazem. The area under the plasma concentration-time curve (AUC) and the peak concentration (Cmax) of diltiazem were significantly increased (P < 0.05, 0.3 and 1.0 mg/kg) with simvastatin. Consequently, the absolute bioavailability of diltiazem with simvastatin (9.9% at 0.3 mg/kg, 11.8% at 1.0 mg/kg) were significantly higher (P < 0.05) than that in the controls (7.4%). Compared to i.v. control, simvastatin did not significantly change pharmacokinetic parameters of diltiazem after intravenous administration. Simvastatin (1.0 mg/kg) significantly (P < 0.05) increased the AUC of desacetyldiltiazem. Moreover, the metabolite-parent AUC ratio (MR) with simvastatin (1.0 mg/kg) was significantly (P < 0.05) decreased compared to that in the controls. This implied that simvastatin effectively inhibited the metabolism of diltiazem. Simvastatin significantly reduced rhodamine 123 efflux via P-gp in MCF-7/ADR cell and CYP 3A4 activity. In conclusion, the enhanced oral bioavailability of diltiazem by simvastatin may result from decreased P-gp-mediated efflux in small intestine and inhibition of CYP 3A subfamily metabolism in small intestine or in the liver. Based on these results, if these results would be confirmed in the patients, the dosage of diltiazem should be readjusted when diltiazem is used concomitantly with simvastatin.