Although it is well established that exogenous androgens have anabolic effects on skeletal muscle mass in humans and mice, data from muscle-specific androgen receptor (AR) knockout (ARKO) mice indicate that myocytic expression of the AR is dispensable for hind-limb muscle mass accrual in males. To identify possible indirect actions of androgens via the AR in neurons to regulate muscle, we generated neuron-ARKO mice in which the dominant DNAbinding-dependent actions of the AR are deleted in neurons of the cortex, forebrain, hypothalamus, and olfactory bulb. Serum testosterone and luteinizing hormone levels were elevated twofold in neuron-ARKO males compared with wild-Type littermates due to disruption of negative feedback to the hypothalamic-pituitarygonadal axis. Despite this increase in serum testosterone levels, which was expected to increase muscle mass, the mass of the mixed-fiber gastrocnemius (Gast) and the fast-Twitch fiber extensor digitorum longus hind-limb muscles was decreased by 10% in neuron-ARKOs at 12 weeks of age, whereas muscle strength and fatigue of the Gast were unaffected. The mass of the soleus muscle, however, which consists of a high proportion of slow-Twitch fibers, was unaffected in neuron-ARKOs, demonstrating a stimulatory action of androgens via the AR in neurons to increase the mass of fast-Twitch hind-limb muscles. Furthermore, neuron-ARKOs displayed reductions in voluntary and involuntary physical activity by up to 60%. These data provide evidence for a role of androgens via the AR in neurons to positively regulate fast-Twitch hind-limb muscle mass and physical activity in male mice.