Mammary glands of virgin, pregnant, lactating, and post-weaning CAF1 mice were studied by immunofluorescence for epithelial immunoglobulins and stromal plasma cells. In normal virgins, both females and males, only occasional plasma cells and but scanty intraepithelial immunoglobulins were present. During pregnancy and the early days of lactation, an increase in the number of plasma cells occurs, and by 1 week of lactation, there is a marked increase in the number, and most are synthesizing IgA. This increment parallels the development and proliferation of the glandular epithelium, in anatomical relation to which the plasma cells are observed. The intraepithelial content of IgA is also maximal when the glandular epithelium is most developed. These findings are consistent with a local production of the IgA in milk. Weaning, or deliberate interruption of suckling for more than 10 days results in a sheep decrease in the number of IgA plasma cells and an involution of the epithelium, whose content of immunoglobulin also declines. The changes in the secretory IgA system which occur locally during lactation are thought to be hormone-dependent.