Adrenocortical zonation and especially border zone formation were investigated in relation to reproduction in female mastomys of the wild-colored inbred strain (MWC). The adrenal weight and the whole thickness of the cortex remained relatively constant throughout pregnancy, delivery and lactation. During pregnancy the zona fasciculata increased, but the z. reticularis decreased in thickness. The border zone between the z. fasciculata and the z. reticularis thickened 2-fold during pregnancy and further 2-fold during lactation. This zone was composed of vacuolated cell layers and nodules in females with a normal reproductive career. In young virgins this zone was barely identified by the presence of a few highly vacuolated cells. These cells became prominent during pregnancy and lactation. During lactation two types of cell aggregations, distinguishable by their stainability, vacuolation, localization and cellularity, appeared in the thickened border zone. The nodules first appeared within the z. reticularis after lactation. Neither thickening of both border zone and z. fasciculata nor formation of nodules occurred when lactation was interrupted after delivery. Thus, the changes in the female MWC adrenal, above all the appearance of nodules, seemed to be more dependent upon lactation than pregnancy. The vacuolated cells and the nodules might originate from the thickened z. fasciculata and the thinned z. reticularis, respectively, by the specific metamorphosis of the cells adjacent to the border zone. Their fusion and induction of fibrous tissue migration may result in the wider border zone.