Early changes in sex hormones are not evident in mice exposed to the uterine carcinogens chloroethane or bromoethane.

Chloroethane and bromoethane have been shown to cause a marked uterine tumor response in B6C3F1 mice exposed for 2 years. These chemicals are nearly unique in this regard among the nearly 400 chemicals studied by the National Toxicology Program, and the reasons for this carcinogenic activity are unclear. The possible relationship of changes in blood concentrations of sex hormones to this response was evaluated by examining the estrous cycle of mice prior to and during a 21-day exposure to concentrations of the haloethanes which resulted in the tumorigenic response in the 2-year studies. Serum concentrations of estradiol and progesterone were determined at the termination of the exposures and compared to exposure group and stage of the estrous cycle. No consistent patterns of change were found in estrous cyclicity or in blood concentrations of sex hormones. Thus, the findings suggest that early changes in circulating sex hormones are not important contributing factors in the uterine neoplasia caused by these chemicals.