In four-day cycling rats studied during the estrous cycle, the levels of circulating norepinephrine (NE) showed a sharp rise concomitant with the LH surge in the afternoon of proestrus. Following the peak, plasma NE fell rapidly to the minimum value by the morning of estrus. The administration of progesterone to ovariectomized estrogen-primed rats resulted in a surge in plasma NE levels 4 h after progesterone treatment. The maximal levels attained were 3-fold greater than baseline values, falling rapidly to reach basal values 8 h after the administration of progesterone. The amine levels remained unaffected 12-20 h later. The LH values attained a maximum 8 h after the injection of progesterone. In rats treated with estradiol benzoate (EB) followed by 0.1 ml of oil instead of progesterone, neither the NE nor the LH peak were observed. These findings are discussed with respect to the significance of increased amounts of plasma NE related to the LH surge.