Purpose: Studying the activity of acid phosphatase, which is the marker of lysosomal activity in the mammary glands of rats at different stages of the physiological maturation [virgih, pregnancy, lactation and Post lactation] Methods: Forty, female, albino rats were used in this study. They were divided into four groups according to their physiological states [virgin, pregnancy, lactation and post lactation]. The mammary glands, after suitable fixation and sectioning, were employed for routine haematoxylin and eosin stain and for acid phosphatase demonstration Results: Acid phosphatase activity was weakly diffuse in the secretory tubules of virgin rats, the diffuse and granular activity of this enzyme was increased during pregnancy in the secretory alveolar cells. The latter cells showed maximum granular activity during lactation. Weak granular with relatively strong diffuse activity of the enzyme was noticed in the alveolar cells of post lactational state. Acid phosphatase activity, in the surrounding connective tissue of the mammary glands of the latter state, was obviously increased in comparing with other experimental groups Conclusion: The lysosomes behave differently in the various phases of the physiological maturation of the mammary glands and as follows: They were inactive in virgin rats They increased during pregnancy and they play a role in tissue remodeling as the organ Grows The lysosomes extensively increased during lactation and they control the secretory processes in the mammary epithelium by fusing with the secretory granules After two weeks of weaning, the