['The objective of this study was to examine the ultrastructural changes in cell organelles such as mitochondria, endoplasmic reticulum (ER) and Golgi apparatus in inflamed colon and uninflamed ileum in colitic rats.', 'Colitis was induced in rats by intracolonic administration of trinitrobenzenesulfonic acid (TNBS). The animals were sacrificed on day 5 after TNBS administration and colonic and ileal samples were used for estimation of myeloperoxidase (MPO) activity, malondialdehyde (MDA) concentration, histologic examination and transmission electron microscopy.', 'TNBS caused a significant reduction in body weight and an increase in MPO activity in colonic, but not in the ileal samples in animals with colitis. MDA levels were increased both in inflamed colon and the uninflamed ileal segments in colitis. Electron microscopy revealed swelling of mitochondria with broken cristae and disruption of the inner membrane. Colitis also caused fragmentation of the ER with loss of ribosomes and swelling of the Golgi apparatus with distended vesicles in both smooth muscle and epithelial cells in the ileal and colonic segments. These changes were absent in the control rats without colitis.'] Is experimental colitis associated with ultrastructural changes in inflamed and uninflamed regions of the gastrointestinal tract?