The use of the standard chemical technic for measuring intestinal glucose absorption was compared with a new double-label radioisotope technic. Glucose absorption and water flux in the mouse small intestine were measured by both methods. The results indicated that the 2 approaches yield almost identical values. The radioisotope technic utilized [14C] polyethylene glycol to measure water flux and [3H] glucose. The technic was found to be reliable, rapid, and applicable to small samples and other substrates. It is particularly suited to absorption studies in small animals such as the mouse, where sample size is limited. The study also showed that the polyethylene glycol recovery rate from the mouse intestine was 97.8%, indicating that it is a valid absorption marker in this species.