Adult female rats show continual vaginal cornification and cease ovulation a few weeks after they are exposed to continuous lighting (light-estrous rats). When these rats were placed in the darkness for 10 hr, 80% of the animal ovulated approximately 46 hr later. Peripheral LH increased to a small peak immediately after placing in darkness concomitant with a decrease in pituitary LH content; a large peak, 20 times higher than the basal LH level, was observed at 20 to 22 hr. Progesterone concentration in ovarian vein blood remained at extremely low levels while estrogen levels tended to rise after small LH peak. This estrogen rise appeared to play an important role in inducing the main LH peak. Simulation of the small LH peak by low doses of exogenous LH succeeded in inducing ovulation of light-estrous rats in similar fashion to the exposure of light-estrous rats to 10-hr darkness. Therefore, the small amount of LH secretion observed after the initiation of the darkness-treatment may be considered as a trigger for the whole sequence of hormonal changes leading to ovulation.