Pregnant and age-matched virgin (AMV) Lewis rats (10 animals each) were used for the experiments. After parturition (at 87 days of age), parous animals were allowed to nurse their offspring for 21 days. At 108 days of age, the litters were removed, and the parous animals underwent 28 days of postlactational involution. At 136 days of age, parous and AMV rats (5 animals each) were sacrificed, and the remaining animals (5 parous and 5 AMV rats) received a single 50 mg/kg dose of Nmethyl-N-nitrosourea (MNU) by an intraperitoneal route. Three weeks after MNU administration, both parous and AMV rats (age, 157 days) were sacrificed.
