BACKGROUND/AIMS: The aim of this study was to investigate the alteration of neuromuscular function and the morphometric changes of the nervous and fibrous tissue in the aged rat colon. METHODS: We used old Sprague-Dawley female rats (n=33, over 20 months), and young rats (n=28, 8-10 weeks). Glass bead expulsion tests were done in vivo, and the colon transit time was measured using a peristaltic bath in vitro. Immunohistochemical staining of PGP-9.5 for the nervous tissue and c-kit for the ICC were performed in the colonic muscle. Collagen quantification was done in each group by employing a spectrophotometer. RESULTS: For the glass bead expulsion, significantly more time was needed for their transit in the young group (p=0.028). The number of rats with complete transit was greater in the young group. The density and area of the nervous tissue and the ICC, as determined by immunostaining were not different between the two groups. On the collagen analysis, a significantly higher level of collagen was detected in the old rats (p=0.02). CONCLUSIONS: The old rats showed a higher incidence of incomplete transit and an increase in their fibrotic tissue. These findings might reflect the degenerative aging process in the colon.