We have investigated the effects of exposure to short-term constant light (LL) on the spontaneous, proestrous luteinizing hormone (LH) surge in the female rat. Exposure to LL during the 3 days preceding proestrus delayed and reduced the magnitude of the LH surge which was measured in blood samples taken from conscious animals through an intra-atrial catheter that had been implanted early on the morning of proestrus. The pituitary responsiveness to synthetic luteinizing hormone-releasing hormone (LHRH) in animals exposed to LL was reduced about 6-fold about the time of the LH surge compared with that in animals on a 14 h light- 10 h dark (LD) regimen. In contrast to animals on LD, treatment of rats on LL with estrogen and progesterone after ovariectomy failed to restore to normal the pituitary responsiveness which is markedly reduced by ovariectomy. The implantation of a silicone-elastomer capsule containing 17 beta-estradiol into rats on LL after ovariectomy did not facilitate pituitary responsiveness. These results suggest that exposure of the female rats to LL for 3 days causes a shift in phase of the LH surge (which may represent a free-running LH rhythm) and reduces the magnitude of the LH surge by a mechanism which may involve a reduction in the sensitivity to estrogen of the centres involved in LHRH release.