To determine the contributions of transporter-mediated and passive absorption during an intraduodenal glucose infusion in a large animal model, six mongrel dogs had sampling catheters (portal vein, femoral artery, duodenum), infusion catheters (vena cava, duodenum) and a portal vein flow probe implanted 17 d before an experiment. Protocols consisted of a basal (-30 to 0 min) and an experimental (0-90 min) period. An intraduodenal glucose infusion of 44 micromol/(kg. min) was initiated at t = 0 min. At t = 20 and 80 min, 3-O-[3H]methylglucose and L-[14C]glucose (L-Glc) were injected intraduodenally. Phloridzin, an inhibitor of the Na+/K+ ATP-dependent transporter (SGLT1), was infused from t = 60 to 90 min in the presence of a peripheral isoglycemic clamp. Net gut glucose output was 21.1 +/- 3.0 micromol/(kg. min) from t = 0 to 60 min. Transporter-mediated glucose absorption was calculated using three approaches, which involved either direct measurements or indirect estimates of duodenal glucose analog radioactivities, to account for the assumptions and difficulties inherent to duodenal sampling. Values were essentially the same regardless of calculations used because transporter-mediated absorption was 89 +/- 1%, 90 +/- 2% and 91 +/- 2% of net gut glucose output. Phloridzin-induced inhibition of transporter-mediated absorption completely abolished passive absorption of L-Glc. We conclude that in dogs, transporter-mediated glucose absorption constitutes the vast majority of glucose absorbed from the gut and is required for passive glucose absorption. The method described here is applicable to investigation of the mechanisms of gut glucose absorption under a variety of nutritional, physiologic and pathophysiologic conditions.