The projections of the medial terminal nucleus (MTN) of the accessory optic system have been studied in the rabbit and rat following injection of 3H-leucine or 3H-leucine/3H-proline into the MTN and the charting of the course and terminal distribution of the MTN efferents. The projections of the MTN, as demonstrated autoradiographically, have been confirmed in retrograde transport studies in which horseradish peroxidase (HRP) has been injected into nuclei shown in the autoradiographic series to contain fields of terminal axons. The following projections of the MTN have been identified in the rabbit and rat. The largest projection is to the ipsilateral nucleus of the optic tract and dorsal terminal nucleus (DTN) of the accessory optic system. Labeled axons course through the midbrain reticular formation and the superior fasiculus, posterior fibers of the accessory optic system, to reach the nucleus of the optic tract and the DTN in both rabbit and rat. Axons also run forward to traverse the lateral thalamus and to distribute to rostral portions of the nucleus of the optic tract in rat only. A second, large projection is to the contralateral dorsolateral portion of the nucleus parabrachialis pigmentosus of the ventral tegmental area together with an adjacent segment of the midbrain reticular formation. The patchy terminal field observed has been named the visual tegmental relay zone (VTRZ). This fiber projection courses within the posterior commissure and along its path to the VTRZ, provides terminals to the interstitial nucleus of Cajal and the nucleus of Darkschewitsch, both bilaterally. A third, large MTN projection distributes ipsilaterally to the deep mesencephalic nucleus, pars medialis, and the oral pontine reticular formation. Further, this projection also supplies input to the medial nucleus of the periaqueductal gray matter, bilaterally in the rabbit and rat, and in the rabbit also to the ipsilateral superior and lateral vestibular nuclei. A fourth projection crosses the midline and courses caudally to reach, contralaterally, the dorsolateral division of the basilar pontine complex and the above nuclei of the vestibular complex. A fifth projection of the MTN utilizes the medial longitudinal fasciiculus to reach the rostral medulla, in which its axons distribute ispilaterally to the dorsal cap, its ventrolateral outgrowth, and the beta nucleus of the inferior olivary complex. There is also a contralateral contingent of this projection that leaves the medial longitudinal fasciculus to innervate a small rostral segment of the contralateral dorsal cap.(ABSTRACT TRUNCATED AT 400 WORDS)