The origin, course and distribution of cerebellopontine fibers was studied in the opossum by employing the Nauta-Gygax and Fink-Heimer techniques. Our results substantiate and extend those of Brodal, Destombes, Lacerda and Angaut ('72) concerning the existence of cerebellopontine projections and provide evidence for a hitherto unreported fastigial projection to the basilar pons. Destruction of the caudal, medial division of the fastigial nucleus elicits bilateral degeneration in a restricted area of the medial pontine nucleus. This small terminal field is located in the angle between the medial lemniscus and the pyramidal tract and is found throughout the caudal three-fifths of the pons. The degenerating fibers do not course within the descending brachium conjunctivum, but reach the pons by filtering through the reticular formation from the uncinate fasciculus. Lesions that involve either the interpositus anterior or the dentate nucleus produce degeneration within the contralateral descending brachium conjunctivum and basilar pons. Terminal fields are located within the median, medial (paramedian nucleus of cat), peduncular, ventral and lateral nuclei. The heaviest degeneration is in the medial nucleus.Although cerebellar and cortical projections have different targets in the basilar pons, there is some overlap. Fastigial and preorbital fibers have partial overlap in the dorsal part of the medial nucleus, whereas the peduncular and lateral nuclei are the areas of overlap between the interpositus anterior and dentate projections with those from forelimb (and probably face) cortical areas. This overlap is particularly obvious in the caudal part of the lateral nucleus and occurs between fibers from limb motor-sensory cortex and those arising mainly within the anterior interpositus nucleus. There is no pontine overlap between cerebellar and visual or auditory cortical projections.In a recent study in our laboratory, fibers from the opossum interpositus and dentate nuclei were traced to their termination within the basilar pons (Martin, '73; Martin, King and Dom, '73). Although most accounts of efferent cerebellar pathways do not mention these projections (see review by Larsell and Jansen, '72), they have been observed in the rat (Mehler, personal communication) and monkey ( Strominger, personal communication), and described in considerable detail for the cat (Brodnl et al., '72). Preliminary examination of our material, however, suggested J. COMP. NEKIR., 154: 257-286. that these connections are particularly extensive in the opossum and furthermore, that an additional projection to the basilar pons arises within the fastigial nuclei. In the light of these findings and because of the potential significance of such connections as part of cerebellar feedback circuits, we have undertaken further study of their origin, course and termination in the opossum.Although Brodal et al. ('72) provided some information regarding possible overlap of cerebellar and cortical projections to the basilar pontine gray, the ob...