What digestive adaptations enable mammals to process much more food in much less time with equal or higher digestive efficiency than reptiles and thus to sustain much higher metabolic rates? To answer this question, we measured glucose and proline uptake in small intestinal sleeves of three mammal and three reptile species of similar body size and natural diet. All species exhibit saturable, stereospecific uptake of D-glucose and Na+-dependent L-proline uptake. Passive permeability to glucose is high in hamsters and low in the other species. Uptake increases with temperature up to a maximum around 45-50 degrees C. This temperature dependence may help explain why reptiles bask after meals and why their digestion is impaired if basking is prevented. The total uptake capacity of the small intestine for glucose and proline is seven times higher in mammals than similar-sized reptiles, mainly because the area of mammalian intestine is 4-5.5 times greater. Minor reasons for the higher uptake capacity of mammals are that the transport activity of mammal intestine normalized to quantity of tissue is up to twofold higher and that reptile intestine operates at a lower temperature at night. Vmax for glucose transport varies 10-fold among species, but apparent differences in Km values may be unstirred-layer artifacts. Carrier-mediated uptake of glucose and proline is measurable in the colon of at least three species, but the uptake capacity of the colon is less than 10% of that of the small intestine. An appendix presents a method for measuring the microscopic area of intestines with ridges rather than villi, applies this method to desert iguana intestine, and measures area amplification due to villi in wood rat intestine.