Ectopic (intrauterine and under the renal capsule) pituitary transplantation induced a high incidence of adenomyosis with subserous nodules in both SHN and SLN strains of mice. Early signs of the development were the invasion of stromal fibroblasts into the myometrium along the branches of blood vessels, followed by uterine gland invasion of the musculature to the serosa. Pituitary grafting did not modify the effects of carcinogens on the uterus. Mechanisms of the development of adenomyosis and hormonal milieu facilitating these pathological changes of the uterus are discussed.