The objective of the present study was to determine the participation of the locus ceruleus (LC) in the regulation of the estrous cycle and of luteinizing hormone (LH) release on the afternoon of proestrus and in ovariectomized rats. Lesions of the LC on the morning of proestrus blocked the preovulatory surge of LH in the afternoon of the same day. The estrous cycle became irregular and erratic after LC lesions and displayed a prolonged period of diestrus and estrus which persisted during the entire experimental period (50 days). After this period the level of LH in the afternoon of proestrus was lower in plasma and higher in the adenohypophysis. A decrease in ovarian, uterine, and adenohypophyseal weight was also observed. LC lesions in 7-day castrated rats induced a decrease in plasma LH levels 48 und 72 h later. When these lesions were made before bilateral ovariectomy, they delayed the normal increase in plasma LH levels. From the data obtained we can conclude that lesion of the LC interferes with the cyclic as well as tonic liberation of LH. A functional rearrangement of the central noradrenergic system may occur a long time after the lesion. These effects may be mediated by connections of the LC with: (1) areas whose neurons produce gonadotropin-releasing hormone; (2) areas which in some form are involved in the liberation of gonadotropins, and (3) the median eminence.