Abstract Background: The Anabolic-androgenic steroids (AAS) are synthetic compounds based on the testosterone hormone structure. They have been abused by athletes to increase the muscular mass. Aim of Study: This study was performed to investigate the biochemical and histological changes in testis of adult albino rat model using different doses of one of the injectable forms of the anabolic steroid nandrolone deconate (ND). Material and Methods: Forty adult albino rats were divided into four groups (ten rats per group), injected IM once a week for ten weeks as follow; group I (control) were injected with 0.1ml of peanut oil, group II, group III and group IV rats were treated respectively with ND in a dose 2.5, 5 and 10mg/kg body weight solved in 0.1ml peanut oil. Results: Results of this study showed that the body weight significantly decreased only in group IV compared to the control group, a significant decrease in the testis weight in group III and IV while insignificantly changes in group II compared to group I. The blood level of testosterone, LH and FSH hormones insignificantly decreased in group II and III but significantly reduced in group IV compared to group I. Histological findings revealed decrease in the number of Leydig cells in the interstitial tissue, change in the shape of the seminiferous tubules, irregularity of the basement mem-brane with some areas detached from the germ cells, The germ cells showed vacuolated cytoplasm and decrease in number with subsequent dilatation of intercellular spaces and finally decreased sperms appearance in the lumen of the seminiferous tubules. These changes were noticed in all groups treated with ND to a variable degree being more evidenced in group III and IV and to less degree in group II compared to the group I. Conclusion: The results of this study showed that fine structures of the testis were affected by ND.