Possible involvement of nitric oxide (NO) in the induction of long-term potentiation (LTP) in the amygdala was investigated using rat brain slice preparations in vitro. The induction of LTP in the medial amygdala was blocked by NO synthase inhibitors, NG-nitro-L-arginine and NG-nitro-L-arginine methyl ester, and an NO scavenger hemoglobin. On the other hand, the lateral amygdala LTP was not blocked by inhibition of endogenous NO. These results suggest that endogenous NO is involved in the induction of LTP in the medial amygdala but not in the lateral amygdala.