The distribution and density of the nonretinal projections to the rabbit medial terminal accessory optic nucleus (MTN) have been studied after injections of horseradish peroxidase (HRP) into the MTN in seven rabbits, and confirmation for the presence of certain of these projections has been made in the rabbit or rat by utilizing anterograde transport of tritiated leucine or leucine/proline after appropriate injections into cerebral cortical areas and brainstem nuclei. In seven cases studied by the retrograde axonal transport method, HRP-labeled neurons have been identified: (A) In four visual or preoculomotor nuclei in which available autoradiographic brain series have confirmed the presence of projections to the MTN: (1) The nucleus of the optic tract/dorsal terminal accessory optic nucleus, (2) the interstitial nucleus of the superior fasciculus (posterior fibers), (3) the periaqueductal gray (including its supraoculomotor portion), and (4) the medial division of the deep mesencephalic nucleus. (B) Within the ventral lateral geniculate nucleus, from which a projection to the MTN has been confirmed autoradiographically in the rat by other workers. (C) In brainstem nuclei and cerebral cortical areas in which available autoradiographic brain series fail to confirm the presence of afferents to the MTN: (1) The nucleus reticularis pontis, pars oralis and pars caudalis, (2) the intermediate interstitial nucleus of the medial longitudinal fasciculus, (3) the nucleus raphe pontis, and (4) five cerebral cortical areas (the area retrosplenialis granularis dorsalis, the striate area, the parietal area 3, the subicular cortex, and the regio praecentralis granularis). Finally, we report retrograde labeling which, on the basis of published connectional data, we believe to result from the spread to and uptake from axons en passant. The false-positive labeling in this category is likely to result from spread of HRP into ventral tegmental nuclei or tracts adjacent to the MTN. Thus, as a result, in the medulla and pons, labeled neurons are found in the medial, lateral, and superior vestibular nuclei, the medullary reticular formation including the nucleus reticularis gigantocellularis, the lateral reticular nucleus, the nucleus raphe magnus, the spinal nucleus of V, the nucleus gracilis/nucleus cuneatus, the dorsal and ventral divisions of the parabrachial nucleus, the central pontine gray, the nucleus K of Meessen and Olszewski, and the dorsal nucleus of the lateral lemniscus.(ABSTRACT TRUNCATED AT 400 WORDS)