Fig. 1. Diagram showing the experimental procedures. Mice entering puberty and exactly 3 weeks of age on the day (Friday) of surgery were anesthetized, their #4 and #9 mammary fat pads were cleared of endogenous mammary epithelium and mammary fragments were implanted immediately into the cleared pads. The mice were allowed to recover for 10 days then the wound clips were removed (Monday). They were injected with 1.0 µg estradiol and ³HTdR i.p., for 5 days, Monday to Friday. On the following Monday, two mice were removed to provide mammary tissues (#3,4,8,9 fat pads) and small intestine for autoradiographical analysis. Thereafter for the next 3 weeks, the remaining mice received 1.0 mg estradiol each Monday, Wednesday and Friday. Then, when they were 10 weeks old (Friday), an additional two mice were removed to provide tissue samples to determine by autoradiography the number of label-retaining cells (LREC) present. On the following Monday, the mice were placed into three groups receiving different hormone combinations and were inoculated with 5BrdU. i.p. 24 hours apart, on Monday and Tuesday. One mouse from each group (3) was removed Wednesday morning to provide tissues for evaluation of 5BrdU incorporation and ³HTdR retention (LREC arrow). The remaining mice (9) received hormones as indicated until Friday and on the following Monday were euthenized to provide tissues for evaluation of 5BrdU incorporation and ³HTdR retention (End chase arrow).