Although studies of the estrogen receptor gene abound in rodent models and breast cancer cell lines, little is known about expression of this gene in normal human breast. Information regarding the physiology of this gene's expression is important if we are to elucidate abnormalities of the gene that may be involved in breast carcinogenesis. We evaluated levels of mRNA expression of the estrogen receptor (ER) gene and its protein product in a set of 89 breasts from clinically normal female infants, children, adolescents, and adult premenopausal and post-menopausal women. mRNA expression of the gene varied with the hormonal status. Relatively higher levels of gene transcripts were found in breasts of peri-menarchal girls, women in the luteal phase of the menstrual cycle, and in those with fibrocystic change. Higher levels were also occasionally found in breasts of infants and in most pre-adolescent children. Lower levels were seen in breasts of women in the follicular phase of the menstrual cycle, during pregnancy, and after menopause. Nuclear protein staining was common in breasts of normal children and peri-menarchal adolescents, and in post-menopausal atrophic breasts. Nuclear ER protein was infrequently detected in reproductive aged women's breasts, but was more often seen in follicular than in luteal menstrual phase or pregnant breast. ER protein was more frequently seen in post-menopausal than in pre-menopausal breasts with fibrocystic change. The results fit a model in which circulating levels of estrogen are inversely related to levels of mRNA transcribed from the estrogen receptor gene in normal physiologic states. Abnormally high levels of gene transcription may occur in some cases of fibrocystic change.