Transit through the bypassed intestine is initially depressed in rats that have 70% of their jejunoileum bypassed. In this study, the influence of intraluminal infusion of the mucolytic N‐acetyl‐L‐cysteine on the depressed transit was determined. Beginning on the first postoperative day, the bypassed intestines were infused continuously with either saline or a 6% solution of acetylcysteine. In one group of animals, transit was determined on the third day after operation. In another group, segments of intestine were evaluated morphometrically on the twelfth day. Transit rates in bypassed intestines of those animals infused with acetylcysteine were higher than those in bypassed intestines of animals infused with saline. Compared to saline infusion, acetylcysteine infusion resulted in the following structural changes in the bypassed intestine: decreases in the total cross‐sectional area of the gut, the wet weight and cross‐sectional area of the intestinal muscle, and the thickness of the circular muscle layer. These findings support the hypothesis that the reduced transit and muscular hypertrophy seen in the uninfused bypassed intestine of rats early after operation may be due to alterations in intestinal contents which incl