The localization of the primary visual centers in the hen diencephalon was determined by anterograde transport horseradish peroxidase (HRP) techniques. Twelve fowls (Gallus gallus domesticus) were used for HRP study and four were used for cytoarchitectural study (Nissl and Klüver-Barrera stained preparation). One-hundred microliter of 30% HRP solution in physiological saline was injected into the vitreous body of one eye of each hen under anesthesia of sodium pentobarbital (30 mg/kg body wt). After a postoperative period of 48 hours, the animals were deeply anesthetized and perfused with an injection of 1,000 ml of Ringer solution which was followed by 2,000 ml of 1% paraformaldehyde and 1.25% glutaraldehyde in a 0.1 M phosphate buffer (pH 7.4) which was then followed by 1,000 ml of 10% sucrose in the same buffer. The brain was cut into serial transverse sections of 60 microns on freezing microtome. Every section was treated with tetramethylbenzidine (TMB). Retinal projections were found in the hypothalamic area, lateral geniculate nucleus (GL), lateral part of dorsolateral thalamus (DLL), medial part of dorsolateral thalamus (DLM), ventrolateral thalamus (VLT), rostrolateral part of dorsolateral anterior thalamus (DLAlr), magnocellular part of dorsolateral anterior thalamus (DLAmc), lateral anterior thalamus (LA), ectomammillary nucleus (EM), external nucleus (NE), and nucleus superficial synencephalica (SS), contralaterally. No labeled terminals were found in the ipsilateral brain stem. In the hypothalamic region, terminals were found to be just lateral to the rostral part of the third ventriculus and the bottom of the lateral margins of the hypothalamus, which we termed medial (MRH) and lateral (LRH) retinorecipient hypothalamic nucleus. LRH had high density terminals compared with MRH and caudal MRH continued into rostral LRH so that there was no boundary between MRH and LRH in HRP preparation. MRH is contained large cells (25-35 microns in diameter) and occupied rostral 1/4 of retinorecipient of hypothalamus (RH), whereas LRH contained small type cells (about 15 microns in diameter) and occupied caudal 3/4 of RH. In the retinorecipient nuclei of the thalamus, high density terminals were found in GL, LA, DLAlr, NE, SS and EM. In DLAlr and EM, granulae of HRP product were bigger than in other terminal nuclei and also the density of those terminals was high. GL and LA have large nuclei which receive retinal afferents. Labeled terminals of those nuclei were distributed homogeneously throughout the whole nucleus except for the inner layer of GL. Cytoarchitectonically, GL was divided into two layers.