Because intratissue levels of estrogens may be more important for the understanding of the endocrine status of an organ than are peripheral plasma levels, the concentrations of estrone and estradiol have been measured in normal and pathological breast and uterine specimens. Some breast tumors were collected in countries with differences in incidence and natural history of the disease. In other samples the subcellular distribution of the steroids was studied. Estrone levels did show much less variation than estradiol levels. Not related to estrogen receptors, estradiol levels were higher in uterine than in breast tissues. Also, the subcellular distribution observed could not be explained by changes in receptors. Malignant breast tumors of premenopausal and postmenopausal women contained similar amounts of estradiol. Unexplained large differences were found in the intratissue estradiol levels obtained in different countries.