Prostaglandin E2 (PGE2) injected into the third ventricle (3rd V) of conscious ovariectomized rats bearing a permanent jugular cannula increased the percentage of plasma samples showing detectable immunoassayable LRF levels at 1, 3 and 5 min after injection. This percentage was small at 2 and 4 min. When plasma LRF and LH titers were measured in animals injected intraventricularly with PGE2 and decapitated 5 min later, both LRF and LH were significantly higher than control values of diluent-injected animals. These results indicate that PGE2 is acting on the hypothalamus to induce release of LRF sufficient in magnitude to be detected by measuring this neurohormone directly in peripheral plasma.