1. The ovaries and uteri of non-lactating mice on the 7th day after parturition are significantly heavier and histologically more active than those of lactating mice.
2. The response to serum gonadotrophin injected on the 2nd day after parturition is of the same order in both lactating and non-lactating mice. This indicates that the ovary is not refractory to gonadotrophin during early lactation.
3. Injection of 5 or 10 i.u. serum gonadotrophin on the 3rd or 5th day after parturition results in normal implantation in lactating mice; injection after 2 days inhibits implantation in both lactating and non-lactating animals, presumably by tubal locking of ova.
4. It has been confirmed that injection of 0·03 μg oestradiol results in implantation at the normal time in lactating mice. There is, however, no indication from the weights of the ovaries of ovarian stimulation, and it is presumed that oestradiol acts directly on the blastocyst, the uterus or on both.
5. It is concluded that the delay in implantation associated with lactation in the mouse is due, at least in part, to reduced secretion of gonadotrophin by the pituitary and the consequent decrease in oestrogen production.