This study investigated whether the circadian regulation of luteinizing hormone (LH) release may be through direct input of the suprachiasmatic nucleus (SCN) to estrogen receptor (ER)- and/or gonadotropin releasing hormone (GnRH)-immunoreactive neurons. We used Phaseolus vulgaris leucoagglutinin (PHA-L) as an anterograde tracer of SCN efferents and performed double label immunocytochemistry for PHA-L and ER or GnRH. Between 8 and 30% of ER cells and 11-13% of the GnRH cells showed appositions with SCN efferents. Efferent projections of the subparaventricular hypothalamic nucleus and the retrochiasmatic area, relay stations of the circadian system, also made appositions with these two cell types. Results suggest that the circadian system could regulate the timing of the LH surge via two pathways, through input to GnRH and to ER cells.