Progesterone and PR are mainly thought to affect tertiary ductal side branching and alveologenesis in late stage of mammary gland development. Here, we present evidence that they also play a role in early ductal development. This conclusion derived from our analysis of maspin heterozygous (Mp+/-) mice that showed defective ductal development at puberty. The defect was due to a reduced systemic level of progesterone. We show that treatment of Mp+/- mice with progesterone rescued the defect of ductal development. When both wildtype and Mp+/- mice were ovariectomized at 4 weeks of age, treatment with progesterone alone can stimulate their ductal growth. In addition, treatment of wildtype mice with the progesterone inhibitor RU486 slowed ductal development in a dose dependent manner. To confirm that progesterone receptor (PR) was required for progesterone action in ductal development at pubertal stage, we treated ovariectomized PR deficient (PRKO) and wildtype mice with progesterone and examined ductal development at 7 weeks of age. While wildtype mammary glands displayed abundant ductal growth after P treatment, there was a significant retardation of ductal growth in PRKO mice. Furthermore, we observed reduced ductal development in intact PRKO mice at 7 weeks of age compared with that of wildtype mice. However, the defect was rescued at late stage of mammary development in PRKO mice. These data demonstrate that progesterone signaling, which is mediated by PR, plays an important role in early ductal development. In PRKO mice, a compensatory mechanism occurs which rescues the ductal defect at a late stage of mammary development.