The mouse mammary tumors developed on the background of changes in the hormonal environment during several cycles of estrus differ from tumors of mice developed during a stable high or stable low hormonal environment. The difference is that the changing hormonal background leads to heterogeneity in the level of estrogen and progesterone receptors in tumors. Female C3H/Sn mice were used in this study. Transplantable mammary tumors were received from spontaneous mammary tumors C3H/Sn mice. ER-positive (ER +) adenocarcinomas in 90% of cases were obtained from mice at the age of 12 months with intensive reproduction (giving birth at least 4-5 times). The level of ER was determined by means of the dextran-coated charcoal technique with the use of 3H-estradiol. The level of receptors protein in mice tumors developed with stable high level of hormones reaches up the range higher than 20,0 fmol/mg of protein up to 110 fmol/mg. The transplanted mice mammary tumors which developed against the environment of high level of estrogens (frequent parturition without mammary feeding of off-springs) contains high level of estrogen receptors. The hormonal environment of the organism determines the level of receptors in the tumors at the end of their development independent of the level of receptors in tumor before transplantation. The wavy development of tumor depending from oestrus phase of cycles during 3-5 cycles is observed in transplanted mice tumors with normal oestrus cycle (cycle changes of hormonal environment). The change of hormones level which is observed during 4-daily of mice oestrus cycle is capable to cyclic influence on change of volume under progressive growth of receptor-positive tumors. The growth loses the wavy feature after 5-th cycle. At the end of tumor development approximately half of cells lose the receptors. Thus, the population become heterogeneous in the content of estrogen receptors. Heterogenization of tumors indicates on the one hand the harmful properties of tumors to adapt to new conditions. On the other hand, the high sensitivity of tumors to changes in the environment gives hope for multi-stage therapy.