Peyer''s patches of adult mice were morphologically studied by light microscopy. The patch was topographically divided into 4 portions: germinal center, follicular area, parafollicular area and dome area. The germinal center consisted of light and dark zones. The follicular area surrounded the germinal center, especially the light zone which had an apical pole directed toward the mucosal surface. The parafollicular area surrounded the follicular area and contained postcapillary venules with high endothelium. The dome area was situated just above the follicular area. The epithelium over the dome area was infiltrated with numerous lymphocytes. Quantitatively, the germinal center, follicular area and parafollicular area were almost similar in volume (about 30%) and the dome area constituted only a small portion (about 10%). The density of small lymphocytes is highest in the follicular area and lowest in the germinal center. Out of the total small lymphocytes in the patch, 8.4% were included in the germinal center; 58.2% in the follicular area; 26.7% in the parafollicular area; and 3.8% in the dome area. Intraepithelial small lymphocytes were almost twice as abundant in the dome as in the villi. In the dome, almost 1/2 the intraepithelial lymphocytes were above the nuclear level of the epithelial cells, whereas in the villi 7.3% of intraepithelial lymphocytes were above the nuclear level. That the functional structure of the Peyer''s patch is similar to that of peripheral lymphatic tissue is suggested.