During chemical carcinogenesis by N-2-fluorenylacetamide, the hormonal status of female Sprague Dawley rats is largely modified. Thyroids present a modified activity which is evidenced by the histological study as well as T4 and T3 assays. In hypophysis, thyrotropic cells were found in an hyperactive state, a fact which is in agreement with the changes observed in thyroid follicles size and epithelial cells demonstrating an hyperactivity of thyroid glands. Despite this histological result, T4 serum level was lowered. T3 level was also decreased but to a lesser extent. We suggest that the low thyroid hormone level play a role in the relative protection of females versus males toward liver cancer induction by chemicals.