The parabigeminal nucleus of the rat was made up of the dorsal, middle and ventral subgroups. When HRP was introduced into the superficial layer of the superior colliculus, labeled neurons were observed in the ipsilateral dorsal and ventral subgroups and in the contralateral middle subgroup. It was demonstrated that neurons of the dorsal and ventral subgroups sent fibers to the ipsilateral superior colliculus, and that those of the middle subgroup sent them to the anterior half of the contralateral colliculus, these fibers being arranged in 'anteroposterior' direction. HRP injections into the pretectal area, lateral geniculate body, suprachiasmatic nucleus and lateral hypothalamic area resulted in the labeling of neurons in the middle subgroup only when the supraoptic commissure and optic tract were injured by a pipette used in injections. Electrolytic lesions of the supraoptic commissure combined with collicular HRP injections resulted in no labeling of cells of the middle subgroup. From these findings, it could be concluded that neurons of the dorsal and ventral subgroups sent axons to the ipsilateral superior colliculus, and that fibers of neurons of the middle subgroup coursed through the supraoptic commissure to the contralateral superior colliculus.