Objective To observe the relationship between nitric oxide(NO) and the cerebral effects of rearing environments in early life Methods The weaning rats were raised either in enriched environmental condition (EC) or in impoverished environmental condition (IC).After exposure,NO synthase (NOS) activity of the hippocampal dentate and cortex was determined quantitatively using NADPH-diphorase histochemistry and Morris maze tasks were tested for each group. Results NOS activity in both dentate gyrus of hippocampus (DEN) and cortex was significantly lowered in EC rats than in IC rats,and the EC rats showed improved spatial cognitive ability as compared to IC rats Conclusions This results suggested that the possible involvement of NO in the cerebral effects of early experience. Administration of L-NAME during environmental exposure failed to alter performances in maze tasks tested after exposure but resulted in lowered NOS activity in DEN and increased NOS activity in cortex.