The effect of progesterone or oestradiol on the delay in implantation in lactating rats was studied with a new technique for local application. Both progesterone and oestradiol abolish the delay. A localized implantation is induced at the normal time by local application of progesterone at around 5 μg, but further increase of the dose up to 100 μg did not increase the area of the uterus affected by the hormone. On the other hand, in the application of oestradiol, the larger the dose, the larger the region of uterus affected by the hormone, resulting in various degrees of responses from local to systemic positive with increase of dose from 0·002 to 0·1 μg.
The cause of the delay in implantation during lactation and possible mechanisms are discussed.