Female rats of the Long-Evans strain, exposed to constant light for 2 months or more and showing constant vaginal cornification, were induced to ovulate by mating. The timecourse of FSH, LH, and prolactin release was followed by measuring plasma levels of these hormones at regular intervals postcoitally. Plasma LH in unhandled constant light-exposed animals resembled diestrous levels in cycling females. Ten min following the onset of mating, plasma LH increased significantly, and reached preovulatory levels by 60 min. FSH titer was within the preovulatory (PM proestrus) range before mating and increased only minimally postcoitally. Plasma prolactin was high in nonmated constant light exposed controls, rose precipitously to a peak 10 min following mating, and remained high for at least 60 min. Nonspecific “stressful” procedures resulted in occasional ovulation, but mean increase in plasma LH was considerably less than following mating. The effects on prolactin of handling and placement in the behavior cage were, however, almost as great as those resulting from mating. Ovariectomized rats, in which plasma LH was suppressed by 5 daily treatments with large doses of estradiol benzoate also showed substantial elevation of plasma LH following mating, but handling procedures had no significant effect in this preparation. It was concluded that the constant light-exposed rat is extremely sensitive to the gonadotropin-stimulating effects of sexual (and other) stimuli; and that mating overcomes the inhibition of LH secretion in spayed estrogentreated females. (Endocrinology93: 1185, 1973)