The vascularly and luminally perfused rat jejunum has been developed as a useful experimental model for the study of intestinal function. However there are discrepancies in the reported results on the fate of luminally absorbed glucose in different versions of this system. In the present work, the vascularly and luminally perfused rat jejunum in vitro was established and thorough investigation was made of the dissection procedure and other experimental variables thought to be important in the functioning of the model. Special attention was paid to the preparation of the erythrocytes for the vascular perfusion medium to ensure their ability to pass through the vascular bed and to deliver oxygen to the intestinal tissues. Measurements of the respiration of the perfused intestine were made routinely in view of the paucity of such observations in the literature. Glucose absorption, translocation and metabolism by the intestine were measured using recirculation and once-through perfusion modes and, in the latter system, analysis of these functions was followed using both enzymatic and radiochemical assay techniques. Glucose and water absorption was also studied using a lumen-only recirculation perfusion of rat jejunum in vivo. The experiments were performed over a wide range of luminal glucose concentrations and osmolarities, and using proprietary glucose-electrolyte solutions intended for use in the oral rehydration of patients. In consequence of this work, it has been possible for the first time to realise why literature reports on the fate of luminally absorbed glucose differ so widely, and the present results appear to give the most accurate record so far on the distribution of this glucose (in the absence of other metabolisable substrates, and using this particular experimental system). It is concluded that the vascularly and luminally perfused rat jejunum in vitro appears to be the best available preparation for the study of the absorptive, translocation and metabolic functions of the small intestine.