Effects of N-nitroso-N-methylurea on mammary and pituitary glands of young female Sprague-Dawley rats.

Female Sprague-Dawley rats aged 50 days received 3 intravenous injections of N-nitroso-N-methylurea, 5 mg/100 g of body weight at 4 weekly intervals. When the first palpable mammary tumours appeared, with an incidence of 50 to 60% after the third injection, rats were randomized into 8 groups: group 1 was treated with 17 beta-estradiol; group 2 received progesterone; group 3 received in association 17 beta-estradiol + progesterone; group 4 was treated with tamoxifen. In group 5, rats received ovine prolactin. In groups 6 and 7, female rats were bred. Group 8, as a control group, received no hormonal treatment. Other control groups of 8 rats aged 140 days were made up of rats receiving 17 beta-estradiol alone (group 9) and rats receiving only ovine prolactin (group 10). The last group was kept without any treatment (group 11). The incidence of mammary tumours was followed in groups 1 to 8. All pituitary glands and mammary tumours were removed and weighed when rats were sacrificed after the last hormonal injection in each group. Light microscopy allowed the anatomopathologist to classify all removed mammary tumours in each group as adenocarcinoma. Light and electron microscopies with all pituitary glands in groups 1 to 11 showed no abnormality. The distribution of the pituitary weights was between 4.0 to 6.8 mg per 100 g of body weight. No adenomas were detected in the pituitary glands of the 5-month-old Sprague-Dawley rats receiving N-nitroso-N-methylurea.