Small bowel transplantation (SBT) has been carried out in man in several cases without success, because immunologic problems were unsolved. In experimental SBT a 'two step' model was developed, which enables long term observation of immunologic phenomena. In this model the graft is in a heterotopic position to the recipient's own small bowel. After 35 days the recipient's own bowel is removed and replaced by the graft, now in orthotopic position and again in contact with luminal chymus. To investigate functional and morphological changes, which result from the procedure, the resorption of glucose and water was measured in syngeneic transplanted rats by an in vivo recirculation system and the mucosa was evaluated three dimensionally. The graft mucosa showed a significant reduction in villus height, crypt length and villus surface and a corresponding decrease in glucose and water absorption during heterotopic position. If the graft came into the orthotopic position, the mucosa did regenerate which was expressed by the significant longer crypts of the graft compared with those of the controls, although the graft's villus height and surface are still smaller. Glucose and water absorption increased and were higher in orthotopic transplanted animals, when absorption was expressed per unit intestinal length. The results indicate that in the 'two step' model of SBT the absorption of water and glucose is influenced to such an extent, that recovery is possible after three weeks, thus enabling orthotopic SBT. This almost complete recovery of the mucosa is further evidence of the regeneratory capacity of the small bowel, which enables clinical small bowel transplantation.