Abstract Because mammary glands are surrounded by adipose tissues, we hypothesize that the ultra-lipophilic endogenous estrogen-17β-fatty acid esters may have preferential hormonal and carcinogenic effects in mammary tissues compared to other target organs (such as the uterus and pituitary). This hypothesis is tested in the present study. We found that all 46 rats implanted with an estradiol-17β pellet developed large pituitary tumors (average weight = 251 ±103 mg) and had to be terminated early, but only 48% of them developed mammary tumors. In addition, approximately one-fourth of them developed a huge uterus. In the 26 animals implanted with a mixture containing estradiol-17β-stearate and estradiol-17β-palmitate (two representative estradiol-17β-fatty acid esters) or in the 29 animals implanted with estradiol-17β-stearate alone (in the same molar dose as estradiol-17β), 73% and 79%, respectively, of them developed mammary tumors, whereas only 3 or 2 animals, respectively, had to be terminated early due to the presence of a large pituitary tumor. Both tumorous and normal mammary tissues contained much higher levels of estrogen esterase than other tissues, which catalyzes the releases of bioactive estrogens from their fatty acid esters. In conclusion, while estradiol-17β is much stronger in inducing pituitary tumor (100% incidence) than mammary tumor, estradiol-17β-fatty acid esters have a higher efficacy than estradiol-17β in inducing mammary tumor and yet it only has little ability to induce uterine out-growth and pituitary tumorigenesis. This study establishes the endogenous estrogen-17β-fatty acid esters as preferential inducers of mammary tumorigenesis.