Replacement of the ovaries of sheep shortly after ovulation with Silastic implants that simulated the physiological patterns and levels of estradiol and progesterone which circulate during the normal estrous cycle produced an “artificial estrous cycle” in which the time course of serum LH and the incidence of estrous behavior closely resembled those of the normal cycle. Neither steroid by itself was effective in this regard. This suggests that two ovarian steroids, estradiol and progesterone, are both necessary and sufficient to account for the secretion of LH and the expression of estrus during the estrous cycle of sheep.
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Karsch et al. (1980) studied this question.