The oestrogenic and antioestrogenic actions of clomiphene were studied in rats. In the absence of oestrogen in ovariectomized animals it exerted a uterotrophic effect, whereas, in the presence of a natural oestrogen (oestradiol) this compound at the same dosage levels counteracted the uterotrophic effect of the oestrogen. In suitable doses clomiphene also counteracted the gonadotrophin suppressing and ovulation-inhibiting actions of oestradiol. The results suggest that this agent may act by competing with the natural oestrogen for the receptor sites. The possible significance of these observations to the mode of action of clomiphene in the induction of ovulation in anovulatory women is discussed.