While carcinogen-induced mammary tumour in rats is one of the representative models for human breast cancer, the age of mammary glands is critical for the induction of tumours by carcinogens; the effects of carcinogens are much marked around 50 days and decline abruptly with the advance of age. Accumulated data have revealed the mitotic rate and the binding activity to carcinogens of mammary glands are major factors in this process, both decreasing with age. Based on these facts, the mechanism of well-known protection by early pregnancy and lactation of human breast cancer and the possibility for prophylaxis of breast cancer were also discussed.