Inseminated female mice were ovariectomized prior to implantation. A daily, dose of 1 mg of progesterone brought about implantation in normal time. Inseminated females suckling a litter of 6 were ovariectomized the 6th day of pregnancy and afterwards given a daily injection of progesterone (0.1 to 10 mg). In no case implantation occurred. A single injection of estrogen (0.5 [mu]g) brought about implantation. Recently implanted eggs were found following the estrogen injection as late as the 10th, 13th or 18th day. It is suggested that a small amount of estrogen is necessary to bring about implantation. This is confirmed by the histological aspect of the ovaries before and after implantation. In non-suckling pregnant females the estrogen secreted by the ovaries is sufficient for sensitizing the uterus. In suckling females no estrogens are secreted and no implantation occurs. It is suggested that the absence of estrogens causes the delay in implantation during lactation.