Micro-anatomical changes in colonic and caecal epithelia of infant rats between birth and weaning appear to be the outcome of the effects of bacterial colonization, superimposed on the natural postnatal ontogeny of the mucous membranes. Conventional rat pups between 15 and 22 days of age showed development of subepithelial spaces in both caecum and colon, rupture of the overlying epithelium and loss of whole enterocyte plaques. Antibiotic-treated animals retained the large-bowel mucosal morphology of the 10-day-old rat, without any of the lesions described in conventional infants. Mucosal lesions were typical of ischaemic damage, possibly triggered by lumen anoxia during colonization.