The sterol content and composition of the small intestinal muccsa has been studied in the rat, guinea pig, and rabbit. In the three species, noncholesterol sterols formed a large proportion of the total sterol content. In the rat, the contribution due to plant sterols was much smaller than in the guinea pig and rabbit, but in all three species the amount of campesterol present seemed disproportionately large compared with beta-sitosterol. These observations are presented as evidence that the intestinal mucosa is able to discriminate in its uptake of sterols from the lumen. The presence of plant sterol at all levels of the intestine makes it clear that luminal sterols can be taken up throughout the small bowel, but the data suggest that the upper intestine is the major site of sterol uptake. In the rat, the sterol composition of the intraluminal contents was also studied after its separation by ultracentrifugation into an aqueous (or micellar) phase and a sediment. Examination of the content and composition of the intraluminal sterols at different levels of the intestine provided support for the idea that the major site of cholesterol absorption was in the upper small intestine. The major part of the intraluminal sterol was found in the sediment; precipitation did not, however, render sterols unavailable for absorption and the micellar sterols reflected closely the pattern found in the corresponding sediment.