Adaptation of the muscularis propria to intestinal resection is not well studied. Growing, 150- to 200-g Sprague-Dawley rats were divided into a resected group (R), which had 75% of the jejunoileum removed; a sham-resected group (SR); and a group of untreated controls (C). Food intake and weight gain were documented. Animals were killed on days 0, 10, 20, 30, or 40 (n = 14 animals for each day in each group). Small-intestinal length and jejunal circumference, wet weight/cm, thickness of the muscle layers, muscle morphometry, mg protein, and micrograms DNA per cm of the muscularis propria were measured. R rats had an initially greater food intake than SR or C animals, but the rate of weight gain was not different between groups. There was no significant increase in length of the residual intestine in R animals during the postoperative adaptive period, but circumference was significantly increased in R compared with SR or C animals. Wet weight/cm, mg protein, and micrograms DNA per cm of the muscularis propria were significantly greater in R compared with SR or C animals, but there was no change in thickness of the smooth-muscle layers in R versus SR or C rats. The number of cross-sectioned circular smooth-muscle cells/mm2 and the ratio of mg protein/micrograms DNA/cm length of muscularis propria were not different in R compared with SR or C animals. Intestinal resection resulted in a significant increase in circumference but not length of the residual intestine. A significant increase in the mass/cm, with no change in the number of cross-sectioned circular smooth-muscle cells/mm2 or in mg protein/micrograms DNA/cm of the muscularis propria, indicates that the adaptive response of muscularis propria is characterized by hyperplasia. This response potentially confers benefit to the organ and animal as a whole as a mechanism that permits structural and functional adaptation of the muscularis propria to resection.