Because fluid absorption values derived from intestinal perfusion may not represent intestinal absorption of the same solution following its oral ingestion, the present study measured intestinal absorption following oral ingestion of a beverage. To do so required the simultaneous determination of gastric emptying. Seven males positioned a nasogastric tube in the gastric antrum and a multilumen tube in the duodenum under fluoroscopic guidance. Gastric emptying (GE) and intestinal water flux (WF) were measured during 85 min of cycle exercise at 60.6 +/- 3.7% VO2max (x +/- SE) in a 22 degrees C environment. Subjects ingested a total of 23 ml.kg-1 body weight (2005 +/- 187 ml) of a 6% isotonic carbohydrate-electrolyte solution by drinking 396 +/- 34 ml 5 min prior to exercise followed by 198 +/- 17 ml every 10 min during exercise. Mean stomach volume (312 +/- 80 ml) and GE (19.7 +/- 2.0 ml.min-1) did not change significantly after the initial 35 min equilibration period. Mean WF during oral ingestion of the solution (19.5 +/- 2.6 ml.cm-1.h-1) did not differ significantly from mean WF (16.4 +/- 1.9 ml.cm-1.h-1) during perfusion of the same solution directly into the duodenum at a rate equal to each subject's GE rate. Total solute flux (mmol.cm-1.h-1) was not different between drinking (4.1 +/- 1.3) and infusion (3.8 +/- 1.0) trials, nor were the changes in plasma volume. Urine production immediately following the exercise bout was unchanged between drinking (89.1 +/- 27.5 ml) and perfusion (88.5 +/- 24.2) experiments. These data indicate that: 1) relatively constant stomach volumes can be maintained over a prolonged period of time and can produce relatively constant GE rates, and 2) intestinal absorption of an isotonic carbohydrate-electrolyte beverage can be accurately determined by a modified segmental perfusion technique employing ingestion rather than intestinal perfusion.