This study was attempted to investigate the pharmacokinetics of phenytoin(4mg/kg iv,) in rabbits pretreated with diltiazem(l and 2.5mg/kg) for 7 days. The plasma concentration and area under the curve(AUC) of phenytoin were increased significantly(p<0.05) in rabbits pretreated with diltiazem(2.5mg/kg) compared with those of control rabbits. Volume of distribution and total body clearance were decreased significantly(p<0,05) in rabbits pretreated with diltiazem compared with those of control rabbits. From the results of this experiment, it is desirable that dosage ragimen of phenytoin should be adjusted and that therapeutic drug monitoring should be practiced for reduction of side or toxic effect when phenytoin will be administered with diltiazem in clinical practice.