Female ACWi (n = 11), ACI (n = 10 contemporaneous, ACIc; n = 63 historic, ACIh) and COP (n = 15) rats were treated with E2, released continuously from subcutaneous Silastic tubing implants, beginning at 9 weeks of age as described in Material and Methods. Thereafter, each rat was palpated once or twice weekly to detect mammary cancer. The data were subjected to Kaplan-Meier analyses. Each plot illustrates the number of days separating initiation of E2 treatment and appearance of palpable mammary cancer or grossly apparent mammary cancer at necropsy. Sham treated control ACWi, ACI and COP rats did not develop mammary cancer over the illustrated time course.