The role of endogenous testosterone(T) in mediating hypertrophic responses to resistance exercise training(RET) remains contentious with correlative studies suggesting a limited role1, and others a more central role2. We thus investigated the physiological and mechanistic effects of chemical endogenous T depletion adjuvant to 6-wk of RET on muscle mass, function, myogenic regulatory factors and muscle anabolic signalling, protein synthesis and glycaemic control in younger men. Sixteen non-hypogonadal healthy young men (18-30y, BMI≤30kg.m-2, serum T>230 ng.dl-1) were randomised in a double-blinded fashion to receive either: placebo (P, saline n=8) or the GnRH analogue, Goserelin (Zoladex (Z) (3.6-mg, n=8)) injections before undertaking 6-wks of RET(6-exercises,3-sets,8-10-reps,80% 1-RM). Participants underwent Dual-energy X-ray Absorptiometry, ultrasound of m.vastus lateralis (VL) and Oral Glucose Tolerance Testing; finally, m.vastus biopsies were taken to quantify myogenic gene expression, anabolic pathways and muscle protein synthesis (MPS) and break down rates (MPB). Provision of Z during RET suppressed endogenous T (Z:45.2±4, vs. P:323±30 ng.dl-1, P<0.001). The Z group exhibited blunted whole-body (Z:55±3 to 56±3, P=0.6 vs. P:56±2 to 58±2kg, P=0.006), appendicular (Z:8.2±0.3 to 8.4±0.4, P>0.9 vs. P:7.6±0.3 to 8.3±0.3kg.m-2, P=0.07), and VL (Z:0.01±0.01 vs. P:6.2±1.1%, P=0.007) lean mass gains, and composite strength gains (Z:40±2 vs. P:50±3%, P=0.03). Reductions in body fat percentage were only evident in the P group (Z:23±2 to 24±2%, P=0.2 vs. P:24±3 to 23±3%, P=0.04). The P group exhibited augmented gene expression related to T metabolism (e.g. Androgen Receptor: 1.8-fold; HSD17B3: 2.8-fold, P<0.05); anabolism/myogenesis (e.g. IGF-1EA (3.3-fold), Myogenin (2.6-fold), Myf5 (2.6-fold), c-Met (2.7-fold), P<0.05) in addition to RNA/DNA (P:0.5±0.1 to 0.6±0.1, P=0.0003 vs. Z:0.5±0.1 to 0.5±0.1, P=0.3), and RNA/ASP ratio (P:6.5±0.2 to 8.9±1.1, P=0.008 vs. Z:5.8±0.4 to 6.8±0.5, P>0.9). Also, acute RET-induced phosphorylation of AKTser473 (Z:4.9±1.6-fold, P=0.7 vs. P:10.1±5.3-fold, P=0.003) and mTORC1ser2448 (Z:2.5±1.3-fold, P>0.9 vs. P:4.8±2.1-fold, P=0.03) were attenuated in the Z group. Both MPS (P:2.01±0.1 vs. Z:1.54±0.1%.day−1, P=0.01) and MPB rates (P:129.1±13 vs. Z:93.1±7.8 g.day-1, P=0.04) were higher in P group, which led to higher net turnover and protein accretion (P:3.9±1.1 vs. Z:1.2±1.1 g.day-1, P=0.04). Only the P group enhanced insulin sensitivity (Cederholm: P:14.5±4.9 vs. Z:2.1±0.4 mg.L2.mmol.L−1.mU−1.min−1, P=0.04). Chemical T depletion during RET blunted muscle insulin/anabolic signalling, T processing enzyme and pro-myogenic gene expression, perhaps explaining attenuated muscle growth and insulin sensitivity in this group. Thus, decreasing endogenous levels of T coupled to RET attenuates numerous positive adaptations to RET – thereby, demonstrating the, at least, permissive importance of endogenous T in regulating muscle hypertrophy.