Diltiazem inhibits calcium channels and Iεads to vascular smooth muscle rεlaxation and negative inotropic and chronotropic effects in the hεart. Diltiazem is almost completely absorbεd after oral administration, but its extent of absolute oral bioavailability is reduced because of considerable first-pass hepatic metabolism. Diltiazem is able to dilate renal vasculature and can increase the glomerular filtration rate and renal sodium excretion. The purpose of this study was to report the pharmacokinetic changes of diltiazem after oral administration of diltiazem, 20 mg/kg, in rabbits coadministered with cimetidine, 20 mg/kg and pretreated twice per day for 3 days at cimetidine dose of 20 mg/kg. The area under the plasma concentration-time curve (AUC) of diltiazem was significantly higher in rabbits pretreated with cimetidine than that in control rabbits (p<0.01), showing about 149% increased relative bioavailability. The peak plasma concentration $(C_{max})$ and elimination half-life of diltiazem were increased significantly (p<0.05) in rabbits pretreated with cimetidine compared with those in control rabbits. This findings could be due to significant reduction of elimination rate constant by pretreated with cimetidine. The effects of cimetidine on the pharmacokinetics of oral diltiazem were more considerable in rabbits pretreated with cimetidine compared with those in control rabbits. The results suggest that the dosage of diltiazem should be adjusted when the drug would be co-administerεd chronically with cimetidine in a clinical situation.