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Daily administration of oestradiol benzoate, beginning 10 days after mating, stimulates lordosis behaviour in deciduomata-bearing pseudopregnant rats, but not in pregnant rats. The inhibition of this behaviour during pregnancy was not prevented by reducing the number of conceptuses to two, by removing the fetuses while leaving the placentas in utero, or by removing the ovaries and administering progesterone to prevent abortion. Removal of the uterus or fetuses and placentas on day 12, however, led to high levels of lordosis behaviour. Thus, it is likely that the placenta produces a factor which inhibits the behavioural responsiveness to oestrogen.

Plasma levels of progesterone, androsterone and dihydrotestosterone were higher during the second half of pregnancy than in the second half of pseudopregnancy prolonged by uterine decidualization. The possible involvement of these steroids in the inhibition of lordosis behaviour was investigated by increasing their levels in deciduomata-bearing pseudopregnant rats and determining the effect on oestrogen-induced lordosis behaviour. Little suppression of this behaviour was seen when the pseudopregnant rats were treated with progesterone or androsterone whereas treatment with dihydrotestosterone resulted in a significant inhibition of lordosis behaviour. However, the dose of dihydrotestosterone required to do so resulted in high, non-physiological plasma levels of this steroid. No inhibition of lordosis behaviour was observed when dihydrotestosterone levels were approximately threefold those normally present in pregnant rats. It is concluded that none of these three steroids is primarily responsible for the suppression of lordosis behaviour during pregnancy.