&NA;Morphologic studies were performed on 25 uteri obtained at different times during the menstrual cycle, ranging from 2 to 140 hours following the onset of menstruation. The amount of endometrium desquamated varied; usually only the more superficial parts—the compacta layer and small areas of the spongiosa— were shed, but in some instances desquamation extended almost to the muscularis. The remaining spongiosa, depleted and shrunken, is misleading in its appearance, resembling a basalis layer. Actually, such a layer appears morphologically distinct only during the initial parts of the cycle and undergoes secretory changes at the same rate as the rest of the endometrium. Desquamation and regeneration coexist, desquamation appearing almost complete by 60 hours, whereas regenerating epithelium is seen as early as 36 hours. Complete regeneration takes place by 140 hours after the initiation of menstruation.