In the rat exposed to constant light, mating reliablyproduces a rapid rise in plasma LH, leading to ovulation. To obtain information on the hypothalamo-pituitary mechanisms involved, changes in LHRF activity of the hypothalamus were studied following mating, and the sensitivity of the pituitary to LHRF was compared in rats exposed to constant light and normally cycling rats. Acid extracts of the medial basal hypothalamus were injected into spayed estrogen-progesterone-treated recipients and the response measured was radioimmunoassayable plasma LH. A decline in LHRF activity of the basal hypothalamus was observed which reached a low point at 20 min after the onset of mating, followed by a return toward the initial level, suggesting rapid release of stored LHRF followed by re-synthesis. No evidence was obtained for altered sensitivity of the pituitary to synthetic LHRF in rats exposed to constant light, in terms of plasma LH response. It was concluded that the particular sensitivity of the rat exposed to constant light to ovulation-inducing stimuli does not result from a high sensitivity of the pituitary to LHRF.