The effect of pinealectomy on the luteinizing function of the hypophysis was studied in sexually mature rats. Following pinealectomy or sham operation rats were blinded in one experimental series, and both blinded and ovariectomized--in another series. Blood LH was determined radioimmunologically with hard-phase immunosorbent produced from antiserum. Pinealectomy does not alter the basic blood LH concentration in the blinded male and female animals. In comparison with the sham operated rats, LH mobilization from the hypophysis was elevated in the blinded ovariectomized rats with removed pineal gland under the action of the maximal LH-releasing hormone dose (200 ng per rat). In comparison with the initial, the hormone concentration increased in control group by 189%, and in the pinealectomized--by 301%. Pinealectomy intensified the autopotentiating action of the LH-releasing hormone (in repeated administration in 60 min) to 1119%, whereas in control rats an increase of the blood LH in response to the repeated adminstration of the LH-releasing hormone constituted 730% in comparison with the initial level. Pinealectomy decreased the inhibitory action of actidion on the autopotentiating effect of the LH-releasing hormone. Besides, pinealectomy accelerated the I125-LH elimination from the circulation in the blinded female rats = 7 min). It is supposed that LH biosynthesis was more active in the hypophysis of pinealectomized rats.