[Modifying effects of pregnancy and lactation on the structure of N-methyl-N-nitrosourea-induced tumors of the mammary glands in rats].

The structure of breast tumors induced by a single i. v. injection of 50 mg/kg methylnitrosourea was compared in virgin rats and those who had been given the carcinogen 30 days prior to onset of gestation. After delivery, progeny were weaned from some animals to stop lactation. In the parous animal group, epithelial component of benign tumors was more pronounced than in virgins whereas malignant tumors showed higher level of cell differentiation.