Although spontaneous peptic ulcers in NZB mice were described first in 1965 and the incidence of the lesion reported two years later no studies of the fine structure of the lesions have been reported. During the course of a scanning electron microscope study of the proximal duodenum of NZB and related mouse strains focal lesions previously unreported were observed. Early lesions were characterised by loss of epithelial cells from the tips of villi. Recently denuded basement membrane had a very rough surface and it was noted that pores 0. 5 to 1. 0 microns in diameter, (Fig. 1) were arranged in rows with regular interspaces between the pores. Epithelial cells which had fallen from the villi in sheets revealing their basement membrane aspect had elongated basal processes (Fig. 2, 3). In a study on the morphology of the human small intestine (Trier, 1968) it was noted that, “Occasionally small pseudopod-like extensions of cytoplasm may penetrate the basement membrane and project into the lamina propria from the basal surface of intestinal epithelial cells”.