The purpose of this study is to ascertain whether or not prostaglandins (PGs) act directly on the pituitary. In the first experiment, the plasma LH levels were sharply decreased 24 hr after the production of hypothalamic lesions in ovariectomized mature rats. Ten min after the iv injection of PGE1, PGE2 or PGF2alpha, the plasma LH levels increased significantly as compared with the vehicle control. Also, the plasma LH levels were elevated following the iv injection of LH-releasing hormone (LH-RH). In the second experiment, after pretreatment of ovariectomized mature rats with estrogen and progesterone, animals received a SC injection of indomethacin, 2 mg in 0.2 ml of gelatin for 3 days. Twenty-four hr after the last injection, 100 ng of LH-RH were injected into the jugular vein. The response of the pituitary to the LH-RH was significantly lower in the indomethacin-treated rats than in the vehicle control rats. In the 3rd experiment, we tested several PGs for stimulatory effects on LH secretion by injecting them into the anterior pituitary. Following direct microinjection of PGE1 or PGE2 (50 or 100 mug/mul) into the rat pituitary, plasma LH was increased at 10 and 45 min after the injection. However, PGF2alpha at both 50 and 100 mug/5mul, failed to increase the plasma LH levels. In the last experiment, the effect of PGE1, PGE2 or PGF2alpha on in vitro rat anterior pituitary LH release was studied. Addition of PGE1, (2 or 20 mug), PGE2 (0.002 or 0.02 mug) or PGF2alpha (200 mug) to the incubation medium produced a significant increase in LH release into the incubation medium. These observations indicate that sufficient dosages of PGs can also stimulate LH secretion by acting directly on the pituitary.