Abstract: Infant Spraque-Dawley rats were divided into several groups and received daily injection of saline,naloxone (10,50,100 or 200 μg/100 g b.w) or Met-enkephalin (MEK 20 μg/100 g 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 days 45,the content of brain protein of the rats was examined.Results showed that the dose of 50 μg/100 g b.w of naloxone significantly inhibited learning behavior in ADL and BDL and lowerd the content of brain protein of the pups.The dose of 200 μg/100 g b.w.of naloxone had the opposite effects.No singnificant effects were seen for the other doses of naloxone.Whereas,MEK inhibited learning of the infant rats in BDL.Results suggest that the endogenous opioid system may influence some behavior during postnatal brain development of infant rats.