The androgen receptor (AR) is widely expressed in mammary cells of mammals indicating a possible role for androgens acting via AR in mammary growth and function. To gain insight into AR functions in the mammary gland, we used global AR knockout (ARKO; Cre-LoxP) female mice and have demonstrated that at 5 weeks of age, ARKO mammary glands displayed accelerated epithelial growth with significantly greater epithelial extension, increased terminal end buds and higher epithelial cell proliferation compared to wild-type (WT) glands. As estrogen receptor α (ERa) is well known to regulate epithelial elongation through TEB proliferation, we assessed if AR inactivation would affect ERa expression. We found that percentage of ERa positive epithelial cells was significantly increased (40.5±5.0 vs 14.2±3.1%, p=0.01; stereology) in the ARKO glands compared to WT. Since both AR and ER may regulate b-catenin signalling relevant for mammary growth, we further investigated if this pathway was also modified by AR inactivation. The percentage of b-catenin positive nuclear cells was significantly increased in the ARKO glands (45.5±9.5 vs 10.2±3.1%, p=0.02) suggesting activation of canonical b-catenin pathway. Therefore, we quantified the mRNA expression (real-time RT-PCR) of Wnt2, 4, 5a, 5b, and 7b in WT and ARKO glands. No significant changes were found in other Wnts analysed except Wnt4, which was upregulated in the ARKO glands (1.8 fold, p=0.03) compared to WT. This may be due to increased ERa in ARKO glands as Wnt4 is regulated by estradiol. No significant differences were found in ovarian estradiol or serum progesterone levels between WT and ARKO females at 5 weeks of age. In summary, our findings suggest that AR-mediated androgen actions suppress mammary gland ERa levels that regulate Wnt4 expression and thereby restricted Wnt/b-catenin signalling to provide a controlled environment for normal mammary growth.