The molecular mechanisms regulating muscle mass changes during immobilisation (de Boer et al. 2007), resistance exercise (Wilkinson et al. 2008) and androgen administration in humans are unclear. We aimed to investigate molecular changes associated with: (i) immobilisation-induced muscle atrophy; and (ii) testosterone-mediated augmentation of muscle mass gains during rehabilitation exercise training in humans. Lean tissue mass (DEXA) was determined in the dominant leg of 12 untrained, healthy, male volunteers (age 20.5±0.6 yrs, BMI 23.6±0.7 kg.m²) before and after 3 wks of limb immobilisation, and at pre-determined time points during 6 wks of isokinetic rehabilitation training (5 x 30 contractions, 3 x wk). Subjects also received weekly intra-muscular injections of vehicle (Control, n=6) or testosterone (600 mg, n=6) commencing immediately following immobilisation. Vastus lateralis biopsies were obtained immediately before and after immobilisation, and after 24 h and 1, 4, and 6 wks of exercise rehabilitation, and were used to determine the expression of proteins involved in the ubiquitin-proteasome pathway and the Akt-mTOR-p70S6k anabolic signalling axis. The study was approved by the University of Nottingham Medical School Ethics Committee. Data are expressed as mean±SEM, and statistical analysis was performed using ANOVA. Lean tissue mass declined by 7.2+1.5% after 3-wks of immobilisation, but this had no effect on the expression of MAFbx and MURF-1 or phosphorylated Akt^Ser473, GSK3β^Ser9, p70S6k^Thr389 and 4EBP-1^Thr37/46. The increase in lean tissue mass from basal was greater in the testosterone group after 4 (8.7±0.8%) and 6 wks (11.0±0.8%) of rehabilitation compared to Control (2.4±0.7, P<0.001; 3.1±0.6%, P<0.001). This testosterone mediated augmentation of lean tissue mass was paralleled by down-regulation of MAFbx (70±80%, P<0.05) and MuRF1 (24.0±12.0%, P<0.05) expression at 6 weeks, and increased phosphorylation of Akt^Ser473 (36±24%, P<0.05; 23±10%, P<0.05), GSK3β^Ser9 (24±11%, P<0.05; 22±10%, P<0.05), and p70S6k^Thr389 (44±22%, P<0.05; 62±31%, P<0.05) at 4 and 6 wks, respectively, compared with Control. This study shows immobilisation induced muscle atrophy was not associated with the upregulation of MAFbx and MURF-1 protein expression in humans, and confirms that anabolic signalling is not blunted under these conditions. Restoration of lean tissue mass during rehabilitation training was augmented by testosterone, and this was accompanied, but was not preceded by, the suppression of MAFbx and MURF-1 expression and increased anabolic signalling.