Vitamin D (VitD) is proposed to have actions upon skeletal muscle. VitD deficiency is prevalent (~25% of the population) and associated with impaired muscle mass, function and metabolism. Moreover epidemiological studies have linked VitD deficiency to age-related sarcopenia (1). In contrast, VitD supplementation has been shown to improve muscle function and increase exercise-induced muscle growth (2). VitD transcriptionally regulates and acts through the vitamin D receptor (VDR), with VDR expression recently being confirmed in muscle (3). The VDR has been linked to muscle regeneration (3), with expression increasing acutely in response to resistance exercise (4). However, results from clinical studies on VitD/ VDR are contentious, with poorly defined mechanistic links between VitD status, VDR and muscle mass/metabolism. We hypothesized the VDR has a functional role in the regulation of skeletal muscle metabolism. To probe the role of the VDR in the regulation of muscle mass, Tibialis Cranialis (TC) of Wistar rats were electroporated (under 2.5% isofluorane and 50mg/kg carprofen) to constitutively over-express (VDR-OE) or under-express (VDR-KD) VDR by cDNA or shRNA lentiviral transfection; contralateral TC’s were sham treated as internal controls. We report here that VDR-OE resulted in myofibre hypertrophy (cross-sectional area (CSA) +17±7%, P<0.05) increased protein content (+57±12%), global protein synthesis (+69±7%, P<0.05) and ribosomal biogenesis (e.g.RPS28 +79%±24%, P<0.05). Comparatively, VDR-KD reduced myofibre CSA (-8±2%, P<0.001), total protein content (-28±16%, P<0.05) and induced autophagy pathways (e.g. LC3B-II +84±43%, P<0.05), without altering global protein synthesis. To investigate clinical context, we sought relationships between lean mass gains and VDR expression in 41 men/women (20-75 y) who underwent whole-body resistance exercise training (RET) – with DXA scans pre/post RET. Following RET, VDR gene expression increased significantly (+11±4%, P<0.05) with gains in lean mass being positively correlated with VDR expression (p=0.02, R2=0.13). Moreover, when grouped into quartiles of lean mass changes, VDR expression tracked responder status i.e. the highest responders increased VDR expression (+20±7%, p<0.05), whereas low responders did not (+8±10%, NS). Similarly, the expression of CYP27B1, a processing enzyme under VDR regulation responsible for synthesizing active VitD, increased only in the high responding groups (i.e. Q1 vs. Q4: +121±8%, P<0.01). Interestingly, serum 1α,25[OH]2D3 status was disconnected from VDR expression and muscle mass regulation. Thus, gain and loss of function of VDR causes hypertrophy and atrophy, respectively in rat muscle while VDR expression and signaling correlates with exercise-induced hypertrophy in humans. VDR expression and not serum VitD status, was the driver of these responses. Thus, the VDR is a potent regulator of muscle mass.