To examine the effects of electroacupuncture (EA) on central nociceptive modulation, expressional changes of spinal neuronal nitric oxide synthase (nNOS) were investigated in complete Freund's adjuvant (CFA)-injected rats. Inflammation was induced by an intraplantar injection of CFA into the hindpaw of male Sprague-Dawley rats. Bilateral EA stimulation at 2 Hz, 15 Hz and 120 Hz was applied at those acupoints corresponding to Zusanli and Sanyinjiao in man with 3-day intervals for 30 days. At 30 days after CFA-injection, effects of EA on nNOS expression were observed in the dorsal horn of the spinal cord using immunohistochemical methods. The mean integrated optical density of nNOS immunoreaction was significantly increased in the dorsal horn throughout L1 to L5 lumbar segments in CFA-injected rats. The nNOS expression was attenuated in all regions of the dorsal horn by all types of EA. Especially, these reaction was markedly decreased in the superficial laminae and nucleus proprius of L1 and L3 lumbar segments by three types of EA, but a marked decrease in the neck of the dorsal horn was observed only in 2 Hz stimulation. The marked decrease of nNOS also showed in nucleus proprius and the neck of L5 lumbar segments in 2 Hz and 15 Hz EA stimulated rats. It is concluded that EA treatment can attenuate chronic inflammatory process in CFA-injected rats through modulating expression of nNOS in the dorsal horn of the spinal cord.