We made an investigation of implantation traces in delivered rats. 1. Nonfixed rat uteri were immersed in 2 %-NaOH solution for over one hour. The uteri were then cleared and the implantation traces were seen to be stained yellowish-brown. This staining method was convenient for observation of the implantation traces. 2. Stillbirth was induced by stabbing or crushing, either in some embryos on one day between the 6 th and 12 th day of gestation, or in all embryos after the 13 th day of gestation. Also, abortion was induced by stabbing or crushing all embryos before the 10 th day of gestation. 3. When live embryos existed in the uterine horns, abortion traces were not detected. 4. In cleared uteri stained with 2%-NaOH solution, abortion traces were observed as small globes, reddish-brown. Normal delivery traces were observed as large globes, yellowish-brown, covered with yellowish-white of agglomerate cells, while stillbirth traces appeared as middle-sized, orange or yellowish-brown masses. 5. Though the implantation traces in rats which had been delivered four times were arranged like beads, they were recognizable by NaOH stain as either old or new traces, the former appearing smaller without agglomerate cells and a little more brown in comparison with the latter. 6. As the implantation traces increased in the uterine horns they became distributed from the central region to the cervical end, and the mean space between them narrowed. The implantation traces occupying the ovarian end or central region of the uterine horns trendes smallest size or largest size. 7. The implantation traces were composed of cicatrical tissue, and the area they occupied did not show adhesion of the placenta. On the other hand, placenta adhering to the uterine wall proliferated, and formed new implantation traces which did not overlap the old traces after decollement of the placenta.