Estrogen is a mitogen in human endometrium and is considered to be responsible also for myometrial cell proliferation. Signalling pathways of estrogen action in these tissues are not known. In various other estrogen responsive cells, estrogen induces transient expressions of c-fos and c-jun mRNAs. We examined c-fos and c-jun mRNA expressions by Northern blotting in paired samples of endometrium, myometrium and leiomyoma tissues obtained from women under various hormonal environments as well as of endometrium and myometrium at term pregnancy. In nonpregnant endometria, strong expressions of c-fos (2.2 kb) and of c-jun (2.7 kb and 3.2 kb) were detected both in the follicular and luteal phase of the menstrual cycle, and the c-fos expression was significantly stronger in proliferative phase endometrium than in the adjacent myometrium. In most of the myometrial and leiomyoma tissue samples the signals for both protooncogenes were weak, and there were no systematic differences in the expressions between normal myometrium and myomatous tissue. In pregnant endometrium and myometrium, both the c-fos and c-jun mRNA expressions were nearly undetectable, and in pregnant endometrium expressions were significantly lower than those in nonpregnant endometrium. Also in late pregnancy myometria, the expression of c-jun was significantly lower than in nonpregnant tissues. These data suggest that c-fos and c-jun activation may be a part of estrogen-induced signal transduction in the endometrium, and that in term pregnancy endometrium this signalling pathway is inhibited. Due to the strong expression of c-jun and c-fos both in the proliferative and secretory phase endometrium, it is likely that these protooncogenes are related to functions other than epithelial cell proliferation in human endometrium. The weak expressions of c-fos and c-jun in the myometrium and in leiomyomata suggest that signalling pathways mediating steroid hormone action in endometrium and myometrium are different.