Different experimental studies suggest that the presence of food in the alimentary tract promote small intestinal absorption. The mechanisms involved are not completely understood and might be related to motility changes or to humoral factors. Since studies have shown a decrease of small bowel motility after casein administration, the aim was to analyze the effect of this protein on small intestinal absorption and orocecal transit time. The hydrogen breath test was used to estimate d-xylose absorption. H-2 production 6 dependent on the amount of this carbohydrate reaching the colon, and therefore inversely proportional to d-xylose absorption. Six normal volunteers ingested 25 g d-xylose and 25 g d-xylose + 30 g casein, and alveolar breath samples were obtained at 15 min intervals. Results: by adding casein to d-xylose solution a statistically significant decrease of maximal H-2 concentration was observed from a mean of 40+/-11 ppm to a mean of 26+/-8 ppm. Similarly the area under the curve (which reflects the amount of xylose that was nos absorbed) was also significantly decreased from a mean of 3281+/-1399 ppm to mean of 1394+/-700 ppm of H-2. The orocecal transit time was significantly prolonged from 85.5+/-40 min to 112+/-38 min. Our results suggest that casein increases d-xylose absorption in normal subjects and that this effect might be related to a slower transit time.