The possible role of substance P (SP) and enkephalin (ENK) was investigated in the mechanism of deafferented pain (DP) and excess pain. The concentration of SP anf ENK was quantitatively estimated with radioimmunoassay using 17 adults cats. The animal were divided into 3 group, DP group (8 cats), EP group (6 cats) and untreated conrol group (3 cats). The DP group was prepared according to the method described by Shimizu and EP model was made by injection 1 ml of Freund's adjuvant subcutaneously into the unilateral face of the cats every day for 3 weeks. The amount of SP in STN markedly decreased in DP group and significantly higher in EP group than with the control group. This is explained by the fact that SP is a neurotransmitter for pain of trigeminal nerve. No significant decrease in ENK in the DP group was observed. This suggests that ENK in STN is not always controlled by the descending inhibitory system, but may be working in the propriospinal system. ENK in the EP group also showed no significant increase. This can be explained not only by the difficulty in estimating the rapid degeneration of ENK, but also by the strong participation of monoamines as well as ENK in pain relief. The role of ENK should not be overstimated and the studies including monoamines will be necessary to detemine the mechanism of pain.