BACKGROUND Diarrhea occurring after small bowel resection gradually improves due to intestinal adaptation. It is known that several water channels, termed aquaporins (AQP), are expressed in the gastrointestinal tract and facilitate water transport. However, the changes of AQP after bowel resection remain unclear. In the present paper, the alterations in AQP mRNA expression were investigated after a massive small bowel resection in the rat residual ileum and colon. METHODS The 6-week-old male Sprague-Dawley rats (n = 15) underwent an 80% distal small bowel resection. The residual ileum and colon were dissected on postoperative day 1, 3, 5 or 7 (n= 3 on each day). Total RNA was purified from each mucosa, and the expressions of AQP and sodium-dependent glucose transporter (SGLT1) mRNA were analyzed by northern blot. The plasma vasoactive intestinal polypeptide (VIP) concentrations on the preoperative day and postoperative day 1 were assayed. RESULTS In the residual small intestine, the expression of AQP-1 and AQP-3 mRNA increased significantly on postoperative day 1. The AQP-7 mRNA increased on postoperative day 3, but the AQP-4 mRNA did not change after the bowel resection. The SGLT1 mRNA gradually decreased after the bowel resection. In the colon, the expression of AQP-3 increased on postoperative day 1 and 7, but AQP-4 mRNA did not change after surgery. The AQP-8 mRNA levels increased slightly on postoperative day 7. Plasma VIP concentration did not change between preoperative day and postoperative day 1. CONCLUSIONS These results indicate that several AQP, except for AQP-4, were up-regulated after a massive small bowel resection, and that AQP might play important roles during adaptation.