Expression of the myometrial gap junction protein connexin-43 (Cx-43) increases dramatically with the onset of labor in association with an increase in the plasma estrogen to progesterone ratio. Moreover, we reported that the levels of messenger RNA (mRNA) and protein encoded by the Cx-43 gene are regulated positively by estrogen and negatively by progesterone in both pregnant and nonpregnant rats. However, although the Cx-43 gene has been reported to be responsive to estrogen, it does not contain a palindromic estrogen response element. It does, however, contain a consensus activator protein-1 (AP-1)-binding site that may be of significance, as estrogen has been reported to increase expression of the AP-1 proteins, Fos and Jun, in the uterus. Therefore, we tested the hypothesis that the increase in Cx-43 mRNA levels in the myometrium during labor and after estrogen administration is mediated indirectly through induction of trans-activating factors, of which Fos and Jun are putative candidates. Treatment of nonpregnant ovariectomized rats with cyclohexamide blocked the estrogen-induced increase in Cx-43 mRNA in the myometrium, suggesting that this action of estrogen requires newly synthesized protein. Estrogen treatment alone induced an increase in mRNA encoding c-fos and c-jun in nonpregnant rats, which preceded by 2-3 h an increase in Cx-43 mRNA. Labor was also associated with a coincident expression of c-fos and Cx-43 in the myometrium, although there was no change in c-jun mRNA levels during this period. These associative data were strengthened by our observation that in paradigms in which labor was induced prematurely by ovariectomy or blocked by treatment with progesterone, changes in c-fos expression were closely matched by changes in Cx-43 mRNA. These data support our hypothesis that estrogen (and, by extension, labor-)-induced increases in Cx-43 mRNA are mediated indirectly through newly synthesized trans-activating factors. Moreover, the temporal correlation between the expression of c-fos and Cx-43 and the presence of AP-1 cis-acting elements within the Cx-43 promoter suggest that c-fos may be one of these trans-activating proteins.