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The responsiveness of the anterior pituitary gland to synthetic luteinizing hormone releasing factor (LH-RF) was tested in rats exposed to constant light. At a dosage of 50 ng LH-RF/ 100 g body wt the mean maximal increments in plasma LH and FSH were similar to those at 10.00 h of pro-oestrus. The increments in the plasma gonadotrophins at dosages of 500 and 1000 ng LH-RF/100 g body wt did not differ significantly from those at 250 ng LH-RF/ 100 g body wt. These findings suggest that, in contrast to rats which exhibit regular oestrous cycles, the preovulatory (post-coital) release of LH in rats exposed to constant light may depend almost entirely on the release of a relatively large amount of LH-RF into hypophysial portal vessel blood. Whereas in pro-oestrous animals a relatively small fraction of the readily releasable pool of LH is released during the spontaneous preovulatory surge, in rats exposed to constant light most releasable LH appears to be discharged during the reflex preovulatory surge of this hormone. The concentrations of radioimmunoassayable FSH in blood samples withdrawn before the injection of LH-RF support the view that FSH secretion in the rat is increased by constant exposure to light.