In the context of developmental biology, puberty emerges as a critical "window of susceptibility," marking a pivotal phase in which the mammary gland undergoes rapid development and becomes highly responsive to physiological and environmental influences, potentially escalating the risk of breast cancer in adulthood. Utilizing the Sprague-Dawley rat model of social isolation, our laboratory has unveiled the previously unexplored phenomenon of mammary development extending into early adulthood, demonstrating that exposure to chronic psychological stress during the crucial period of early puberty through early adulthood (5-21 weeks) impedes maximal ductal extension growth and alveolobular differentiation. Our investigation delves into the lingering question of how heightened exposure to chronic psychological stress correlates with decreased mammary development and differentiation.