The purpose of this study was to assess the effect of cobalt on the reproductive structure andhematology of adult male Rhode Island Red chickens (Gallus gallus domesticus). Sexually mature chickenswere given single dose of 30 mg/kg body weight cobalt chloride intraperitoneally for 48 hours. The gain inbody weight was not significantly different between control and treatment group, however the testicular weightwas significantly reduced (P<0.001). Histopathological evaluation of the testes revealed several abnormalitiesincluding degeneration of spermatogonial cells spermatocytes, spermatids and necrosis of seminiferoustubules. Analysis showed that the diameter of seminiferous tubules was significantly decreased (P<0.05).Evaluation of the number of RBCs and WBCs detected significantly (P<0.0001) increased in experimentalgroup. Biochemical analysis of plasma showed that there was no significant difference in glucose, cholesteroland urea concentration. Contrary to this, the biochemical analysis of reproductive organs showed significant(P<0.0001) decrease in glucose concentration while cholesterol and DNA level increased significantly(P<0.0001) and urea concentration remain unaltered. Results of this study report a negative effect of cobalton hematology and histology of testes