Mammary-tumour incidence in Sprague-Dawley rats treated with 7,12-dimethylbenz(a)anthracene: effect of pregnancy and lack of effect of unilateral lactation.

Mammary teat removal (thelectomy) was performed unilaterally in female Sprague-Dawley rats at 35 days of age. They were given 7,12-dimethylbenz(a)anthracene (DMBA) when aged either 55 days or 79 days. One third were unmated; one third were mated one week and one third mated more than 3 weeks after DMBA administration. Animals were killed when tumour-positive or after one year, when mammary lesions had developed in 99% of rats. The mean latent period for adenocarcinomas was 18.9 +/- 2.0 weeks. Benign mammary tumours, mainly secretory adenomas, developed significantly later (39.2 +/- 1.7 weeks). The rapid unilateral involution of the thelectomized glands at parturition had no effect on the localization of either adenocarcinomas or benign mammary tumours. Pregnancy and delayed DMBA administration markedly reduced the incidence of adenocarcinomas; lactation had no significant effect. In a separate experiment, precocious puberty induced with pregnant-mare-serum gonadotrophin in 30-day-old female Sprague-Dawley rats enabled their first pregnancy and lactation to be completed by 80 days of age. Parity before carcinogen administration significantly delayed the development of adenocarcinomas.