Acute administration of 17 beta-estradiol (5 micrograms/rat) to 25-day-old female Sprague-Dawley rats resulted in an increase of uterine mRNA for the cellular oncogene c-fos. The c-fos mRNA levels were significantly elevated 12 and 24 h after exposure to the hormone (232 and 164% of control values) and the elevation was not observed after 48 h. In contrast, treatment of the animals with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) resulted in a dose-dependent decrease in constitutive uterine c-fos mRNA levels. In rats co-treated with 17 beta-estradiol plus TCDD or MCDF, it was apparent from the results that the halogenated aromatic hydrocarbons significantly inhibited the estrogen-induced increases in uterine c-fos mRNA levels. These observations further extend the diverse spectrum of antiestrogenic effects caused by TCDD and related compounds and also show an interaction between TCDD and the constitutive expression of the c-fos proto-oncogene in the female rat uterus.