Recently, we have published that treatment of pituitary isografted BALB/c mice with a single injection of N-methyl-N-nitrosourea (MNU) leads to the rapid development of mammary tumors in over 90% of the animals (Guzman et al., Cancer Res., 52, 5732-5737). In the present study, we characterized the changes in proliferative activity and lobulo-alveolar differentiation of MECs at different time intervals after isografting animals with pituitary glands. Virgin BALB/c mice 1, 3, 5 or 8 weeks after pituitary isografting were either pulse-labeled for 2 h or continuously infused with bromodeoxyuridine (BrdU) and the percentage of BrdU-labeled MECs was assessed. The S-phase duration (TS) of MECs was evaluated by double labeling with [3H]thymidine and BrdU. The population potential doubling time (TP) was calculated from the values of BrdU-LI and TS. Three stages of proliferation and differentiation of MECs in pituitary isografted virgin BALB/c mice were observed: (i) A sharp increase in the percentage of proliferating MECs of the terminal ducts and ductal branchings in the first 1-2 weeks, (ii) Development of lobulo-alveolar structures from the terminal ductal and alveolar buds, between weeks 3 and 5 with the highest BrdU-LI in week 3 and (iii) Multiplication of the alveolar structures and decrease in the BrdU-LI between weeks 5 and 8. The BrdU-LIs of alveolar cells 5 weeks after isografting the animals were significantly higher than those of the ductal cells. The continuous administration of BrdU for 3, 5 or 7 days by using osmotic pumps revealed zones in the ducts where almost all MECs were labeled as well as zones lacking proliferate activity. When the BrdU administration was extended for 10-14 days, almost all (> 95%) ductal and lobular epithelial cells were labeled. A small percentage (< 5%), of ductal and lobulo-alveolar MECs cells, remained unlabeled even after 14 days infusion of BrdU. The TS and TP values were shorter in pituitary isografted animals than in controls, but no significant difference was found for either values between the ductal and alveolar cells in either isografted or control mice. Changes in proliferation kinetics of mouse MECs in pituitary isografted animals correlated with the circulating concentrations of prolactin, progesterone and 17 beta-estradiol, but not with corticosterone, growth hormone or thyroxin.(ABSTRACT TRUNCATED AT 400 WORDS)