Abstract: Nitric oxide(NO) content and nitric oxide synthase(NOS) activity were measured in cerebral cortex isolated from Wistar rats of three age groups(young: 3 months; adult: 9 months; and old: 27 months). No significant differences in NO content and NOS activity between young and adult rats were found(P>0.05). The NO content and NOS activity in old rats were shown to be significantly higher than those of young and adult rats(P<0.01). When treated with Rg1(10, 20, 40 mg·kg ^-1), the NO content and NOS activity in old rats decreased. The inhibitory effect of Rg1 on NOS was found to be dose dependent in the range of 10～40 mg·kg^-1. The optimal reduction in NO content and NOS activity induced by Rg1 occurred at 40 mg·kg^-1 for old rats(P<0.01). In view of the close relationship of NO content and NOS activity with aging, the inhibitory effect of Rg1 on NOS activity, as shown by our results, might provide an explanation for its antiaging function.