The acute effects of low doses of clomiphene citrate upon serum LH within the first two hours after the intravenous injection into ovariectomized, estrogen-progesterone blocked rats were investigated. There was a short-lasting elevation of LH release 45--60 min after injection of 2.5 microng clomiphene. A second LH releasing effect occurred within 60 min when 50 microng clomiphene or more were injected. Doses between these two dose levels were not effective. It is concluded that these effects may probably be due to a partial reduction of distinct estrogen receptor sites both in the hypothalamus and in the pituitary which causes a short-lasting decrease of the negative feedback of the circulating estrogens.