Abstract: The cytochrome P- 450 ( Cyt P-450)sensitive to mepyramine (Hp-450 ) in liver microsome is a protein that possesses properties of both H₁ receptor and cytochrome P-450.We used ³Hmepyramine radioligand binding assay and enzymological technique to study the kinetic properties of HP-450 and Cyt P-450.Age-related changes in the Bmax,Kd of HP-450 and Cyt P-450 in rat hepatic microsome were demonstrated.The levels of Bmax for both HP-450 and the Cyt P-456 dramatically increased in the postnatal period from the second to the eighth week ,and reached maximum steady status during the eighth to tenth week.The values of Bmax of HP-450 correlated well with the content of Cyt P- 450 in rat liver microsome(r=0.625).The affinity of HP-450 (Kd) decreased with the growth of rats during postnatal. No sexual-related difference was observed in this experiment.