Maltose and lactose absorption, which require an intact brush border for breakdown and absorption as glucose, was evaluated as a function test to monitor the integrity of the small-bowel graft. Using the rat model of accessory small-bowel transplantation, absorption tests (in the form of an oral glucose tolerance test) were performed on iso- and allografts with either portal (PP-A) or caval venous drainage (PC-A). In isografts the absorption of maltose was found to be reproducible and not influenced by the type of venous drainage. This was not the case with the use of lactose which thus was not studied further. Allografts with PC-A demonstrated a reduction in their capacity for maltose absorption on the fifth postoperative day, as the glucose peak at 30 min (T30) was significantly blunted in comparison to that for isografts with PC-A (167 mg% +/- 12 vs 204 mg% +/- 8). Functional impairment preceded histologic changes which did not arise before the sixth-to-seventh postoperative day in rats with PC-A. Allografts with PP-A absorbed maltose on the fifth postoperative day nearly as effectively as did isografts (T30 min: 185 mg% +/- 14 vs 213 mg% +/- 8). By the ninth postoperative day, the serum glucose curve after maltose administration was flattened for grafts with PC-A (T30 min: 137 mg% +/- 11) which were rejected acutely (host's death) after 11.8 days +/- 0.45. A similar impairment of maltose absorption was not seen in the PP-A group (chronic graft rejection after 22.8 days +/- 1.8) until the 15th postoperative day.(ABSTRACT TRUNCATED AT 250 WORDS)