The cyclic changes in the mammary gland of the unmated mouse are described; there is no luteal phase in this cycle, growth of the gland culminating at estrus. The macroscopic changes occurring in the mammary gland during pregnancy, lactation, and involution are described: growth of the mammary gland continues after parturition, until the peak of development is reached at about 12 days post-partum; regression is complete by about 12 days after weaning. The microscopic changes occurring during pregnancy, lactation, and involution are also described, and a new phase in regression consisting of the formation of rounded masses from the remnants of one or more nuclei associated with a large globule, presumably of fat, is noted. The changes in the mammary gland during pseudo-pregnancy are described: the peak of development reached is comparable with the condition at 9 days pregnancy. Comparison is made with the conditions existing in other species, notably the albino rat, rabbit, and guinea pig.