Over the course of 1 year the Ka value of uterine oestrogen receptor for oestradiol varied 10-fold from a low of 0.259 .+-. 0.065 .times. 109 M-1 in early spring to 2.21 .+-. 0.21 .times. 109 M-1 in fall. There were no significant changes in receptor number. In addition, administration of oestradiol or progesterone to ovariectomized rabbits resulted in significant reductions in measurable oestrogen receptor affinity. Although these variations in oestrogen receptor Ka values parallel reproductive success in the domestic rabbit, no causal relationship has been estalished.