The authors examined the neuro-effector transmission in the aganglionic segment from 13 patients with Hirschsprung's disease, 10 patients had short-segment type and three patients had long-segment type. Circular muscle strips were prepared and the responses to transmural electrical field stimulation were examined using the isometric tension recording technique. In the ganglionic preparations from the short- and long-segment cases, the stimulation evoked a biphasic response consisting of a relaxation and an atropine-sensitive contraction. The relaxation was partly inhibited by N-omega-nitro-L-arginine (NNLA, a nitric oxide [NO] synthase inhibitor), and the effect of NNLA was abolished completely in the presence of L-arginine, which suggested the presence of NO-mediated inhibitory innervation. In the aganglionic preparations from the short-segment-type cases, stimulation evoked only an atropine-sensitive contraction, while in the aganglionic preparations from the long-segment-type cases, a weak inhibitory response persisted after the contractile response was abolished by atropine. This NO-mediated inhibitory response was frequently detected as the examined region approached the ganglionic segment. These results suggest that the aganglionic segment in the long-segment-type cases might therefore receive NO-mediated inhibitory input from the proximal ganglionic segment.