The purpose of this study was to investigate the effect of morin, a flavonoid, on the pharmacokinetics of diltiazem and one of its metabolites, desacetyldiltiazem in rats. Pharmacokinetic parameters of diltiazem and desacetyldiltiazem were determined after oral administration of diltiazem (15 mg/kg) in rats pretreated with morin (1.5, 7.5, and 15 mg/kg). Compared with the control group (given diltiazem alone), pretreatment of morin significantly increased the absorption rate constant $(K_a)$ and peak concentration $(C_{max})$ of diltiazem (p<0.05, p<0.01). Area under the plasma concentration-time curve (AUC) of diltiazem in rats pretreated with morin were significantly higher than that in the control group (p<0.05, p<0.01), hence the absolute bioavailability $(AB\%)$ of diltiazem was significantly higher than that of the control group (p<0.05, p<0.01). Relative bioavailability $(RB\%)$ of diltiazem in rats pretreated with morin was increased by 1.36- to 2.03-fold. The terminal half-life $(t_{1/2})$ and time to reach the peak concentration $(T_{max})$ of diltiazem were not altered significantly with morin pretreatment. AUC of desacetyldiltiazem was increased significantly (p<0.05) in rats pretreated with morin at doses of 7.5 and 15 mg/kg, but metabolite-parent ratio (MR) of desacetyldiltiazem was decreased significantly (p<0.05), implying that pretreatment of morin could be effective to inhibit the CYP 3A4-mediated metabolism of diltiazem. There were no apparent changes of $T_{max}$ and $t_{1/2}$ of desacetyldiltiazem with morin pretreatment. Collectively, the pretreatment of morin significantly altered pharmacokinetics of diltiazem, which can be attributed to increased intestinal absorption as well as reduced first-pass metabolism. Based on these results, dose modification should be taken into consideration when diltiazem is used concomitantly with morin or morin-containing dietary supplements in clinical setting.