In order to investigate the effect of testosterone on the immunological activity of thymus gland, the specific activity of adenosine deaminase enzyme (ADA) in albino male mice was evaluated. The mice were divided into two age groups (4 and 5 weeks of age). Each age group was sub-divided into three subgroups: hypersecretion group, normal group and control group. The group of hyper – testosterone secretion was injected with 0.1 mL of the hormone with the concentrations of 5,25,50,75,150 and 200 mg for 14 days and then compared with the normal group (untreated animals) and control group (animals treated with 0.1 mL of sunflower oil for the same periods of time). The results demonstrated the testosterone had a direct effect on the activity of ADA in serum, thymus and thymocytes homogenates in both groups and the most significant decrease (P≤0.0001) was observed at the concentration 200 µg /0.1 mL.