Androgens promote anabolism in the musculoskeletal system while generally repressing adiposity, leading to lean body composition. Circulating androgens decline with age, contributing to frailty, osteoporosis, and obesity; however, the mechanisms by which androgens modulate body composition are largely unknown. Here, we demonstrate that aged castrated rats develop increased fat mass, reduced muscle mass and strength, and lower bone mass. Treatment with testosterone or 5α-dihydrotestosterone (DHT) reverses the effects on muscle and adipose tissues while only aromatizable testosterone increased bone mass. During the first week, DHT transiently increased soleus muscle nuclear density and induced expression of IGF1 and its splice variant mechano growth factor (MGF) without early regulation of the myogenic factors MyoD, myogenin, monocyte nuclear factor, or myostatin. A genome-wide microarray screen was also performed to identify potential pro-myogenic genes that respond to androgen receptor activation in vivo within 24 h. Of 24 000 genes examined, 70 candidate genes were identified whose functions suggest initiation of remodeling and regeneration, including the type II muscle genes for myosin heavy chain type II and parvalbumin and the chemokine monocyte chemoattractant protein-1. Interestingly, Axin and Axin2, negative regulators of β-catenin, were repressed, indicating modulation of the β-catenin pathway. DHT increased total levels of β-catenin protein, which accumulated in nuclei in vivo. Likewise, treatment of C2C12 myoblasts with both IGF1Ea and MGF C-terminal peptide increased nuclear β-catenin in vitro. Thus, we propose that androgenic anabolism involves early downregulation of Axin and induction of IGF1, leading to nuclear accumulation of β-catenin, a pro-myogenic, anti-adipogenic stem cell regulatory factor.