The purpose of this study was to assess the effects of cobalt on the testicular structure of adult golden hamsters. Hamsters in group A received cobalt (CoCl2) in single intraperitoneal dose 20 mg/kg, in group B 10 mg/kg and in group C 5 mg CoCl2/kg body weight and were killed fourty eight hours after cobalt administration. Afer a preparation of histological samples the results were compared with control. After a cobalt administration dilatation of blood capillaries in interstitium, undulation of basal membrane and occurrence of empty spaces in seminiferous epithelium was detected. Morphometric analysis showed that in all cobalt-treated groups the relative volume of seminiferous epithelium was significantly decreased. In the relative volume of interstitium a significant increase was found between control group and experimental groups. After cobalt administration we have found linear non-significant decrease. Evaluation of diameter seminiferous tubules found increase of this parameter in the all experimental group in comparison with control. Height of seminiferous epithelium was relatively constant and in all groups but the difference between control and group A was significant (P < 0.05). Analysis of the lumen diameter of seminiferous tubules detected significantly increase mainly group B. Evaluation of the number of cell nuclei per a constant area detected an increase of this parameter in experimental group. Results of this study report a negative effect of cobalt on structure and function of testes.