The objective of this study was to investigate the effect of prepubertal injection of recombinant bovine somatotropin (rbST) on physical and biochemical semen characteristics of Rahmani lambs fed different levels of dietary energy. A total of twelve Rahmani lambs were used in this study. The initial live weights were 30.9±1.03 kg, (5-6 months old). The lambs were randomly divided into three symmetric experimental groups, the first group (n=4) were treated with Nacl 0.9% and fed a complete mixed

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... ration containing 72.0% TDN to cover 100% of their requirements (Normal Energy, Control). The second and the third groups (n=4) were treated with 100mg rbST/14days, but the second group fed on the same diet of the first group (NE-rbST), while the third group fed a complete mixed ration containing 86.4% TDN to cover 120% of their requirements (High Energy, HE-rbST). The lambs were allowed 15 days to adapt to energy diets. The amount of feed was calculated to cover their allowances according to NRC (1985) and modified to cope with body weight changes. After two months from the last dose of rbST, semen samples were collected twice weekly from each ram for nine weeks. Semen volume, sperm motility, abnormalities, live sperm percentage, sperm concentration, sperm total output, total live sperm were recorded. Seminal plasma was separated after centrifugation and stored at -20 • С until analysis. Ejaculate volume increased significantly in NE-rbST group compared with HE-rbST, but did not differ significantly from the control. Ejaculate volume did not differ significantly in HE-rbST from the control rams. The NE-rbST group had a lower (P<0.05) semen density score than the control, but did not differ significantly compared with HE-rbST group. Sperm concentration increased (P<0.05) in HE-rbST group compared with NE-rbST and the control. There were no significant differences in sperm concentration between NE-rbST and the control groups. Treatment of rbST did not cause any significant effect on sperm mass motility, sperm advanced motility, percentage of live sperm, total live, sperm abnormalities and sperm output in all experimental groups. Seminal plasma concentration of total protein, globulin, triglycerides, total lipids and activities of AST, ALT, AST/ALT ratio and A/G ratio were not changed significantly in all experimental groups. Seminal plasma concentration of albumin decreased (P<0.05) in HE-rbST compared with NE-rbST, but did not differ significantly from the control. Seminal albumin concentration did not differ significantly in NE-rbST compared with the control. Seminal plasma concentration of Hafez et al. 142 cholesterol increased (P<0.05) in HE-rbST compared with the control, but did not differ significantly from NE-rbST group. However, seminal plasma cholesterol concentration in NE-rbST group did not differ significantly compared with the control.