The monoamine innervation of the organum vasculosum lamina terminalis (OVLT) was studied using fluorescence histochemistry and the autoradiographic tracing method. Fluorescence histochemistry demonstrates serotonin terminals in close proximity to the capillary plexus of the OVLT, in the same region where the apparent terminals of luteinizing hormone-releasing hormone neurons are localized. No catecholamines are evident in this area. Autoradiographic studies indicate that the serotonin innervation of the OVLT arises in the midbrain raphe nuclei. Thus, the OVLT is similar to other circumventricular organs, the subfornical organ and the subcommissural organ, in receiving a rich innervation from serotonin neurons.