This study examined whether six days recombinant human growth hormone (rhGH) administration, 0.058 IU/kg/day, in an abstinent anabolic-androgenic steroid (AAS) using group had any respiratory, cardiovascular and biochemical effects compared with an abstinent AAS control group. Impairment in respiratory function in adult-onset growth hormone deficiency (AO-GHD) is consequential to a reduction of respiratory muscle strength, responding to replacement therapy with rhGH (Merola et al., 1996). RhGH significantly improves exercise tolerance in cystic fibrosis (CF) (Hutler et al., 2002) and significantly improves respiratory function in major surgery, and is more beneficial when given pre- and post-operatively than post-operatively alone (Barry et al., 1999). Male subjects (n=48) were randomly divided, using a double blind procedure into two groups: (1): exercise control group (n=24, mean ± SD, age 32 ± 11 years; height 1.8 ± 0.06 metres); (2): rhGH using group (n=24, mean ± SD, age 32 ± 9 years; height 1.8 ± 0.07 metres). Anthropometry, peak oxygen uptake and respiratory muscle function were investigated. Respiratory measurements examined, were forced expiratory volume in one second, forced vital capacity (FEV1/FVC), MIP and maximum expiratory pressure (MEP). Cardiovascular measurements were blood pressure (BP), heart rate (HR) and rate pressure product (RPP). Biochemical analysis included; glucose, sodium, urea, creatinine, total protein, albumin, testosterone and insulin like growth factor-I (IGF-I). FEV1/FVC (85 ± 6 vs. 82 ± 5, %), MIP (144 ± 24 vs. 129 ± 28, L), MEP (179 ± 35 vs. 157 ± 32, L), resting HR, (78 ± 11 vs. 67 ± 16, bpm) resting RPP (97 ± 14 vs. 84 ± 24, bpm.mmHg X 10-2) and IGF-I (323 ± 93 vs. 169 ± 50, ng/ml) significantly increased compared with the control group (all P<0.05). Body mass index (27.7 ± 3.1 vs. 27.5 ± 3, kg.m-2), fat-free mass index (22.3 ± 1.9 vs. 21.9 ± 1.9, kg.m-2), peak oxygen uptake (45.4 ± 9.9 vs. 41.8 ± 9.8, ml.kg-1.min-1), MIP (144 ± 24 vs. 131 ± 30, L), MEP (179 ± 35 vs. 165 ± 36, L), IGF-I (323 ± 93 vs. 159 ± 54, ng/ml) and serum sodium (141.8 ± 2.5 vs. 140.6 ± 2.6, mmol/L) significantly increased, whilst body fat (19 ± 6 vs. 20 ± 6, %), total protein (73.1 ± 4.5 vs. 75.7 ± 4.9, mmol/L) and albumin (42.5 ± 4 vs. 44.4 ± 4, mmol/L), significantly decreased within the GH group (all P<0.017). The findings of this study indicated that short term high dose rhGH increased aerobic performance and respiratory muscle strength in former AAS users, but may have an adverse effect on the cardiovascular system, as evidenced by the increase in resting rate pressure product. Acknowledgements: Acknowledgements to Mr Christiaan Bartlett, King’s College, London, for analytical work.Reference 1 : Merola B, et al. Lung volumes and respiratory muscle strength in adult patients with childhood- or adult-onset growth hormone deficiency: effect of 12 months' growth hormone replacement therapy. Eur J Endocrinol 1996;135:553-558.Reference 2 : Hutler M, et al. Effect of growth hormone on exercise tolerance in children with cystic fibrosis. Med Sci Sports Exerc. 2002;34:567-72. Reference 3 : Barry MC, et al. Nutritional, respiratory, and psychological effects of recombinant human growth hormone in patients undergoing abdominal aortic aneurysm repair. JPEN J Parenter Enteral Nutr 1999;23:128-35.