Chronic administration of estradiol leads to generation of estrogen-dependent renal cancer in male Syrian hamsters within 9-12 months. The pathogenesis of this tumour is associated with oxygen free radicals, however, it is still not clear which metabolic pathway of estrogens is responsible for the generation of the radicals, and which part of the cell is mostly affected by it. The paper presents an experiment in which the protein oxidation measured by carbonyl groups level in 9-month estrogenization scheme. The level of carbonyl groups was significantly elevated starting from the 1st month of estrogenization until the 9th. The weight of the kidneys reflecting possible tumuorigenesis was also significantly increased in estrogenized group starting at the 2nd month of the experiment. The weight of the testes, a sign of adequate estrogenicity, was decreased dramatically in the estrogenized group from the first to the last month of experiment. The results suggest that the kidney of estrogenized male Syrian hamster suffers from oxidative stress affecting proteins.