The levels of substance P (SP) in rat brains were assayed in 64 rats. Bilateral common carotid artery ligation was done in 49 rats. Half an hour before ligation, 25 rats were given 10 g/kg of RSM; 24 rats were given the same volume of normal saline as controls. Sham operation was done in 15 rats. Half an hour and 3 hours after cerebral ischemia, the rats were quickly decapitated. SP concentration was assayed in the cerebral cortex, caudate nucleus and brain stem. In saline-treated animals, the SP level of caudate nucleus at 3-hour group was significantly decreased as compared with the 0.5-hour group and sham-operated group respectively. No significances were found among RSM-treated groups and sham-operated groups. The SP levels were shown: brain stem greater than caudate nucleus greater than cerebral cortex. The preliminary results suggest that SP may be involved in the pathophysiologic procedures of cerebral ischemia and RSM may attenuate the dysfunction of SP during cerebral ischemia.