Differences in intestinal absorption among physiological states, intestinal regions, and animal species could arise from many anatomical factors (e.g., intestinal length and diameter, or area amplification by villi and microvilli) and biochemical factors (e.g., transporter density and turnover number). In no comparison had all these factors been measured. Hence we made the necessary new measurements to identify the origin of differences in glucose absorption among three species (mouse, desert wood rat, and desert iguana) and among three intestinal regions in two of these species. Turnover numbers range from 6,900 to 32,300 glucose molecules per minute per site. Microvilli amplify intestinal area by a larger factor (36-96 times) than do villi (2-14 times), so that the intestine's actual area is at least 110-1,280 times the nominal area of the equivalent smooth-bore cylinder. Species comparisons among mammals yield the striking result that the area of the whole length of the small intestine at the microvillus level varies nearly linearly as the mammal's metabolic live mass. For the species studied, all the anatomical and biochemical factors studied proved to make significant contributions to species and regional difference in glucose uptake.