Precise and accurate light microscopic morphometric analyses of biological tissue can be achieved utilizing component quantitative techniques. Component quantitation refers to measurements of the relative volumes of components in tissue sections. Such an assessment is predicated upon the mathematically verifiable assumption that direct quantitative relationships exist between an aggregate of profiles of a component contained per unit area in multiple sections and an aggregate of profiles contained per unit volume. A linear scanning device (micrometer component quantitator) was initially employed for quantitative analyses of pancreas. This quantitative technique has subsequently been applied to normal rat ileum conventionally processed for light microscopy, and the requisite sampling parameters have been defined. An identical technique was then applied to physiologically manipulated rat ileum--a gnotobiotic group, a group with ileal self-filling blind loops, and a group with ileal Thiry-Vella loops. The results observed support the following conclusions. The volume percentage of the various components of the rat ileal wall of control animals was defined utilizing the micrometer component quantitator. Hypertrophy of the ileal muscularis externa within the ileal self-filling blind loops was observed, probably secondary to mechanical obstruction. Atrophy of the ileal epithelium within the gnotobiotic group and within the Thiry-Vella loops was observed, possibly secondary to an altered endogenous microbial flora. Recognition of quantitative variations among the histological components of the intestinal wall in association with physiological manipulations or pathologic states was (is) feasible by utilization of this component quantitative technique.