Following 0, 5, or 18 h of cold ischemia in the University of Wisconsin or Sacks' solution syngenic small bowel transplantation was performed to the renal vessels. Mucosal injury during ischemia or at reperfusion was evaluated microscopically using a grading scale from 0 to 8. The series subjected to small bowel transplantation without cold ischemia showed superficial damage to the tips of the villi (median grade 2) with no further exacerbation at reperfusion and complete healing 24 h thereafter. Moderate villous injury (median grade 3) was demonstrated following 5 h of cold ischemia. This injury was exacerbated to grade 4 (p less than 0.05) following reperfusion but was almost completely healed 24 h later. Grade 4 tissue injury was seen following 18 h of cold ischemia without further exacerbation at reperfusion and no clear tendency to healing 24 h later.