The vertebrate dorsal mesencephalon consists of the superior colliculus, the dorsal portion of the periaqueductal gray, and the mesencephalic trigeminal neurons in between. These structures, via their descending pathways, take part in various behavioral responses to environmental stimuli. This study was undertaken to compare the origins and trajectories of these pathways in the cat. Injections of horseradish peroxidase into the cervical spinal cord and upper medullary medial tegmentum retrogradely labeled cells mainly in the contralateral intermediate and deep superior colliculus, and in the ipsilateral dorsal and lateral periaqueductal gray and adjacent tegmentum. Only injections in the medullary lateral tegmental field labeled mesencephalic trigeminal neurons ipsilaterally. Autoradiographic tracing results, based on injections across the dorsal mesencephalon, revealed three efferent fiberstreams. A massive first fiberstream (limbic pathway), consisting of thin fibers, descended ipsilaterally from the dorsal and lateral periaqueductal gray and adjacent superior colliculus through the mesencephalic and pontine lateral tegmentum, terminating in these areas as well as in the ventral third of the caudal pontine and medullary medial tegmentum. A few fibers from the dorsal periaqueductal gray matter (PAG) were distributed bilaterally to the dorsal vagal, solitary, and retroambiguus nuclei. The second fiberstream (the predorsal bundle) descended contralaterally from the superior colliculus (SC) and consisted of both thick and thin labeled fibers. The thin fibers terminated bilaterally in the dorsomedial nucleus reticularis tegmenti pontis and the medial half of the caudal medial accessory inferior olive. The thick fibers targeted the contralateral dorsal two thirds of the caudal pontine and medullary medial tegmental fields, and the facial, abducens, lateral reticular, subtrigeminal, and prepositus hypoglossi nuclei. A few fibers recrossed the midline to terminate in the ipsilateral medial tegmentum. Caudal to the obex, fibers terminated laterally in the tegmentum and upper cervical intermediate zone. From the lateral SC, fibers terminated bilaterally in the lateral tegmental fields of the pons and medulla and lateral facial subnuclei. The third fiberstream (mesencephalic trigeminal or Probst tract) terminated in the supratrigeminal and motor trigeminal nuclei, and laterally in the tegmentum and upper cervical intermediate zone. In summary, neurons in the PAG and in the deep layers of the SC give rise to a massive ipsilateral descending pathway, in which a medial-to-lateral organization exists. A similar topographical pattern occurs in the crossed SC projections. The possibility that these completely different descending systems cooperate in producing specific defensive behaviors is discussed.