Murine implantation experimental design. The inguinal mammary glands of 3-week-old Balb/C mice were cleared of host epithelium and inoculated with either 20,000 (NSP2) or 50,000 (NSP3, SP3) immortalized, premalignant cells. Control (thoracic) number 3 and 8 glands remained intact. 5-BrdU was administered for 2 consecutive days/week for 5 weeks, followed by a break (chase) period of 3 weeks. Nulliparous female mice were then inoculated with 3H-TdR and subsequently euthanized within 90 minutes. The remaining mice were mated, inoculated with 3H-TdR, and euthanized within 90 minutes of receiving label at either 4 to 6, 8 to 10, or 12 to 15 days after coitus.