OBJECTIVE: To investigate the effect of spinal cord injury (SCI) on contractions of whole colonic preparation isolated from rats under the inhibition of nitrergic inhibitory neural system using tetrodotoxin (TTX). METHOD: Twenty Sprague-Dawley rats were used. A complete spinal cord transection was performed surgically at the T10 cord level in spinal cord injured group. After 1 week of operation, sensory and motor functions were assessed and colon was removed under anesthesia for in vitro motility study. Whole colon was divided into four segments: proximal, two mid colon and distal colon. Each segment of colon was mounted with longitudinal direction in a 10 ml organ bath. After 1 hour of equilibration, frequency, area under the curve of spontaneous contraction and the response to acetylcholine (Ach), KCl and TTX were measured in each segment. Also the responses to Ach and KCl response under TTX pretreatment were measured. RESULTS: Enhanced contractile response to KCl solution (40 mM), TTX (1 micrometer) and Ach (10(-6) M) was observed in both group. There was no statistical difference in spontaneous, Ach and KCl induced contraction between control and SCI rats, but TTX induced contraction was decreased in SCI group than control group (p<0.05). In addtion, the Ach and KCl responses under the TTX pretreatment were significantly decreased in SCI group than control group (p<0.05). CONCLUSION: These results suggest that the change of colonic contractility after the SCI is caused by at least partly from the change of TTX related inhibitory neural system.