The terminal areas and the cells of origin of the projection from the sensory trigeminal nuclei to the mesencephalon were investigated, using the method of anterograde and retrograde transport of horseradish peroxidase or wheat germ agglutinin-horseradish peroxidase conjugate. Injection of tracer into the nucleus interpolaris or nucleus oralis (in the latter cases with involvement of the nucleus principalis) resulted in dense anterograde labeling in the deep and intermediate gray layers of the contralateral superior colliculus, extending throughout the rostrocaudal extent of the colliculus with the exception of its caudalmost part, which was not labeled. Minor projections to the intercollicular nucleus, posterior pretectal nucleus and nucleus of Darkschewitsch were found. Injection of tracer into the nucleus caudalis yielded a completely different result; terminal labeling in the midbrain was now present only in the periaqueductal gray matter, in its rostral and middle parts. The retrograde labeling observed after injection of tracer into the midbrain terminal areas showed that the cells of origin were located mainly in the alaminar spinal trigeminal nucleus, and the highest density of labeled neurons was found in the rostral part (subnucleus y) of the nucleus oralis. The retrograde labeling in the nucleus principalis was very sparse and almost exclusively involved peripherally located neurons. In the nucleus caudalis the overwhelming majority of the retrogradely labeled neurons were situated in its marginal layer. The functional implications of the above observations are discussed in relation to the findings in previous studies of the projections from the dorsal column nuclei and spinal cord to the midbrain. The combined results suggest that the trigeminal projections to the superior colliculus may be involved in the mechanisms of orientational behavior. The observation that the projection to the periaqueductal gray matter originates in the marginal layer suggests that it transmits information related to noxious stimuli.