The neuro-endocrine mechanisms involved in the olfactory block to ovo-implantation in mice were studied by investigating methods of preventing the block from taking place. Daily administration of 50 i.u. of prolactin to females on Days 1 to 5 is fully effective in preventing the block to pregnancy. Pregnancy block can also be inhibited in normal females by the presence of a functioning ectopic pituitary homograft, which serves as an additional source of prolactin. Reserpine is also effective in inhibiting the block to pregnancy, thus providing a direct proof of hypothalamic intervention in the reaction. The inhibition of the pregnancy block by exogenous progesterone and the histological appearance of the corpora lutea give further support to the view that implantation failure is due to failed luteal function. The results reported in this study provide additional evidence in favour of the view that olfactory stimulation from males causes the failure of prolactin secretion and initiates the secretion of fsh and lh by stimulating the hypothalamus. This, in turn, causes the failure of corpus luteum development and the return of the female to oestrus in the ordinary way.
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