The aim of this investigation is to study the rote of alkaline phosphatase in mammogenesis and lactogenesis. A total of fortyfemalealbino rats were used and divided according to their physiological states into four groups [ten rats each]. From each deeply ether anesthetized rat, the mammary gland was removed, fixed, quenched in liquid nitrogen and sectioned using SLEE cryostat. The sections were employed for routine haematoxylin and eosin stain and alkaline phosphatase demonstration using the calcium–cobalt method. The obvious finding in the mammary glands of pregnant rat was the presence of thick black rings indicating strong alkaline phosphatase activityaround the basal part of the secretory epithelium of the alveoli. In lactating mammary glands, the black rings were thin, discontinuous and limited to the basal part of the secretory epithelium. On the other hand, the mammary glands of virgin and post-lactating rats exhibited no such enzymatic activity around the basal part of secretory tubules and involuting alveoli respectively. The strong enzymatic activity observed around the secretory epithelium of alveoli in pregnant and lactating mammary glands indicating that the basement membrane and myoepithelial cells of the alveoli were very well recognized and developed in these groups than in virgin and post–lactating ones