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The influence of local serotonin injection into the preoptic region (PR) or arcuate nucleus-medial eminence (ARN-ME) on the hypophyseal and blood luteinizing hormone (LH) level was studied in experiments on intact female rats. It was shown that a serotonin inhibiting effect is limited by a definite time interval of the proestrus phase, whereas within other phases of the estrous cycle this monoamine exerts the opposite action on LH secretion (the morning time of diestrus 1 and diestrus 2 stages), testifying to the modulating serotonin role in the control of the hypophyseal gonadotropic function. In experiments on ovariectomized female rats, some of which were given estradiol benzoate, it was shown that serotonin injection into PR was accompanied by a decrease in the blood LH level only in ovariectomized rats, treated with estradiol, and did not change LH secretion in sterilized female rats. Serotonin stimulation of ARN-ME region lowered the LH level only in ovariectomized rats and did not influence LH secretion in the animals given estradiol.