Small intestines from normal adult rats were quantified by optical and electron microscopy using stereological principles devised for the purpose. Five segments per bowel were examined. Baseline data characterising villi, microvilli and crypts of Lieberkühn were used to study differences between segments and between animals. Intestines fixed by in situ perfusion had, on average, 100 cm2 of primary mucosa. This basic surface was amplified to 500 cm2 by villi and to 1 m2 by the microvilli of enterocytes. Villous and microvillous surface areas may scale to body weight in the same way as metabolic requirements. Proximodistal gradients in mucosal architecture existed for the volumes and surface areas of villi and for the numbers, lengths, diameters and surface areas of microvilli. Most variables were higher proximally and declined towards the terminal ileum. The volume of crypts stayed constant throughout the entire intestine and ratios between villous dimensions (volumes and surface areas) and crypt volume did not vary between animals. Findings are discussed in the context of regional differences in bowel function and of their relevance to studies of epithelial kinetics.