Active stress and cross-sectional area of intestinal muscle were assessed in tissues taken from unoperated rats, from rats that had undergone bypass of 70% of the small bowel, and from rats that had undergone transection and anastomosis of the bowel. Thirty-five days after operation, muscle from the intestine of transected and bypassed animals elicited active stresses that were equal to or greater than those developed by muscle taken from unoperated animals. The total cross-sectional areas of the in-continuity segment and the area and thickness of the muscle layers of both the in-continuity and bypassed segments were greater when compared with unoperated animals. Significant differences also existed among tissues taken from bypassed and transected animals. Additionally, transection induced increases in active stress, area of muscle in the distal intestine, and circular muscle thickness in the mid- and distal intestine when compared with tissues from unoperated animals. These findings support the hypothesis that intestinal bypass induces increases in functioning smooth muscle tissue.