The serum level of intravenously dripped phenytoin was deter- mined in rabbits before and in different time intervals of a week after intramuscular injection of diazepam with fluorescence polarization immunoassay and the parameters were calculated on a microcomputer.It was found that in the 12th hour after diazepam injection,the serum level of phenytoin was significantly different from that of the control,and t1/2 of phenytoin was significantly prolonged;the model of pharmacokinetics of phenytoin was changed from one compartment open model to two compartment open model.Yet the AUC and Cl of phenytoin were not markedly altered.