Standard doses of -125-I-labelled rat IgG were injected into the intestinal lumen of rats aged 16 days, and their sera were sampled 2 and 3 hr later. High concentration quotients were obtained after injection into the proximal small intestime, whereas very little immunoglobulin was transmitted from doses injected into the terminal 20 cm of the small intestine. 2. The villi of the terminal 18--20 cm of the small intestine of 16-day-old rats, the region from which very little transmission of IgG occurred, were lined by tall columnar absorptive cells with very larg supra-nuclear vacuoles. The extent of the terminal intestine, in which this cell type predominated in the absorptive epithelium, varied with age. The importance of defining the precise location of the region of the intestine under examination is stressed. 3. The experimental results and the histological observations are discussed in relation to (a) the results which have been obtained using PVP, which is unsuitable as an indicator of immunoglobulin transport in the rat and (b) the histological composition of the absorptive epithelium and the maturation changes which affect the epithelium between 18 and 21 days.
