We performed this study to develop a reliable method for inducing colitis in rats using 2,4,6-trinitrobenzene sulfonic acid (TNBS) to reduce the variation in ulcer size. A pair of ring forceps was used to clamp the colon and 0.1 M TNBS in ethanol was injected into the luminal side of the clamped portion. This method resulted in a small coefficient of variation of the ulcer index. A significant linearity was observed by plotting ulcer size against days after ulcer induction in both logarithm scales. These findings show that this technique is simple and reliable and that ulcers heal linearly.