Mammary tumours in female rats developed after 1-methyl-1-nitrosourea treatment

Female Sprague-Dawley adult rats were administered intraperitoneally with 5 doses of 50 mg.kg(-1) body weight 1-methyl-1-nitrosourea (MNU) in the firs t experiment and with 2 doses (one dose per two months) of MNU in the secon d experiment. On the Iii day in the first experiment and on the 153(rd) day of age in the second experiment rats were killed by decapitation and their mammary glands were evaluated morphologically for the presence of mammary tumours. All excised tissues were stained with hematoxylin and eosin and we re evaluated histochemically for the presence of actin and S-100 protein. A ll microscopically evaluated tumours were malignant. Of the total number of lesions classified the percentage of invasive tumours was in the range of 35-44%. No metastases were observed in other organs.