BACKGROUND: The ileum has a greater adaptive capacity than the jejunum after intestinal resection. Transposition studies suggest that this is, in part, related to increased exposure to nutrients. However, there may be regional differences in intestinal properties that contribute to this response as well. The aim of this study was to compare the outcome of replacing the jejunum with either ileal or jejunal transplants while preserving an intact ileum after 50% proximal resection. METHODS: Twenty-one Lewis rats were included in the study. One group (n = 7) served as transection controls (TC). The other two groups (n = 7) had 50% proximal resection with syngeneic transplantation of a similar length of ileum (ITX) or jejunum (JTX). Nutritional status and adaptation were studied at 14 days. RESULTS: JTX animals gained less weight than TC and ITX (104 +/- 2% vs 114 +/- 1 and 108 +/- 2% initial, P < 0.05). ITX and JTX groups had lower caloric intake and serum albumin levels compared with TC (7.4 +/- 0.4 and 7.2 +/- 0.8% vs 8.8 +/- 0.2% body weight and 2.6 +/- 0.1 and 2.4 +/- 0.9 g/dl vs 3.0 +/- 0.1 g/dl, P < 0.05). Mucosal thickness increased significantly in the ileal remnant of both ITX and JTX groups (9.2 +/- 2.1 and 8.8 +/- 0.6 micrometer vs 6.6 +/- 0.6 micrometer, P < 0.05). Transplanted ileum had mucosal thickness similar to that of jejunum. CONCLUSIONS: Transplanted ileum achieves an intestinal structure similar to that of the jejunum in the same environment. Modest adaptation of the remnant occurs with transplantation. Replacing jejunum with ileum rather than jejunum resulted in better weight gain, suggesting that intrinsic absorptive, motor, or hormonal rather than structural differences are responsible.