&NA;Oxygen radicals play a key role in inflammation and inflammatory tissue damage. Quantitative determination of pentane, a hydrocarbon generated by membrane lipid peroxidation initiated by oxygen radicals, in expired air has been used as a noninvasive determinant or index of inflammation in various conditions. Herein we report the first examination of the relationship between exhaled pentane and colonic inflammation in a rodent model of colitis. Colitis was induced in rats (n=33) using the trinitrobenzene‐sulfonic acid (TNB) model of colitis. Exhaled air was collected in a closed chamber on randomly selected animals on days 1, 2, 4, 7, 11, 13, 15, 20, and 25 post‐TNB treatment, and pentane was assayed by means of gas chromatography. Gross and microscopic evidence of inflammation was compared with exhaled pentane levels. Pentane levels varied from 0.0 to 14.6 nmol/l of air and were significantly increased in TNB‐treated rats compared with control rats only on days 7 to 15 after treatment(P<0.05). Gross inspection showed severe colonie inflammation through the first week (mean score =4.7 out of a possible 5), persistent inflammation on days 7 to 15 (3.2), and healing and fibrosis from the end of week two until day 25 (1.9 to 0). Histologic evaluation confirmed a progression of inflammation from acute ulceration to chronic inflammation to fibrosis and scarring. We have demonstrated that pentane exhalation is increased after the induction of colonie inflammation, with a seven‐day lag time, and returns rapidly to normal as acute inflammation resolves. This suggests that pentane exhalation can be used as a noninvasive measure of colonic inflammation in rodent models of colitis and perhaps clinically in humans.