Abstract: Infant Spraque-Dawley rat pups were divided into three groups and received daily injection of saline,bestatin〔an aminopeptidase inhibitor,1 mg/kg(b.w)〕or naloxone〔2 mg/kg(b.w)〕＋bestatin〔1 mg/kg(b.w)〕respectively from postnatal days 1-14.They were trained in the appetitive discrimination learning (ADL) at the age of 16 days,then in the bright discrimination learning of Y-maze (BDL) at 30 days of age.On postnatal days 45,the content of brain protein and zinc in cerebral cortex,hippocampus,cerebellum,pons,and diencephalon of the rats were examined.The results showed that the dose of bestatin inhibited significantly the learning behavior in ADL and BDL,and lowered the content of protein and zinc in hippocampus of the infant rats.Whereas,naloxone reversed partly the effect of bestatin.This suggested that the endogenous opiotide whould regulate some learning behavior through influncing synthesis of protein in hippocampus during postnatal brain development of infant rats.