Induction of sexual receptivity by oestradiol benzoate in cyclic female rats: influence of ovarian secretions before injection of oestradiol benzoate.

The ability of cyclic female rats to show sexual receptivity 24 h after an injection of 2 microgram oestradiol benzoate (OB) was lost 24 h after ovariectomy. Exposure of cyclic rats to antioestrogen (nitromophene monocitrate) implants 24 h before ovariectomy and OB treatment prevented the latter from inducing sexual receptivity within 24 h of administration. Treatment of ovariectomized rats with constant release implants filled with an oil solution of 15 microgram oestradiol/ml had no behavioural effect in itself, but prepared the rats to show lordosis 24 h after administration of OB. Progesterone treatment (4 mg) induced sexual behaviour in cyclic rats on days other than that of the oestrous cycle when the rats are normally receptive. Evidence is presented that a lower level of oestradiol stimulation than that present duing pro-oestrus was needed for the induction of sexual receptivity in ovariectomized rats. It is suggested that the low basal level of oestradiol which was present throughout the oestrous cycle was necessary for the induction of sexual receptivity and that an increase in oestradial stimulation served to increase the behavioural sensitivity to progesterone.