The thyrotropin-releasing hormone (TRH)-containing neurons that project to portal capillaries of the external zone of the median eminence (ME) and fenestrated capillaries of the organum vasculosum of the lamina terminalis (OVLT) were identified on thin paraffin and thick vibratome sections using a combination of retrograde labeling with peripherally administered Fluoro-Gold and fluorescence immunocytochemistry. The results indicate that the vast majority of those TRH neurons that project to the ME and the OVLT is located in the paraventricular nucleus (PVN), and most abundantly, in its medial parvicellular subdivision. Although numerous TRH-immunoreactive (TRH-i) neurons are present in other hypothalamic areas of the brain, only a few of them in the dorsal hypothalamic area behind the PVN and the periventricular preoptic nucleus could be retrogradely labeled. Since only a few Fluoro-Gold-accumulating and TRH-i perikarya were seen in other nuclei than the PVN, it is likely that the majority of nerve terminals in the OVLT also originates from TRH-i perikarya in the PVN. Fluoro-Gold, an electron-dense substance, is stored in the lysosomes of hypophysiotropic TRH-i perikarya and thus, it provides an excellent model for electron microscopic characterization of hypophysiotropic neurons at both the light and electron microscopic levels. The data together provides additional morphological evidence for the key role of the PVN in the regulation of TSH secretion.