A double-labeling method which employed the injection and subsequent retrograde transport of two different fluorescent dyes was utilized to investigate the possibility that some pontocerebellar neurons might give rise to collateral branches which distributed to more than one lobule of the contralateral lateral cerebellar hemisphere (intrahemispheric branching) or bilaterally to homotopic or heterotopic lobules in both lateral hemispheres (interhemispheric branching). With regard to intrahemispheric branching, these studies revealed the largest number of double-labeled neurons when the dye injections involved a combination of crus I and the paraflocculus. A considerable number of double-labeled cells were also observed in simplex-crus II cases while a modest number were noted in crus I-paramedian and simplex-paramedian combinations. Evidence for interhemispheric branching was also apparent but the number of double-labeled cells was generally less than that observed in the intrahemispheric experiments. Following bilateral injections, a modest number of double-labeled cells was noted with simplex-simplex and crus I-Crus I homotopic injections and crus II-paramedian heterotopic combinations. In contrast to those cases with unilateral injections, not all bilateral injection combinations produced double-labeling, the most conspicuous in this regard being homotopic and heterotopic injections involving the paraflocculus. Overall, the population of double-labeled cells included many varieties of pontine neurons ranging from small, spindle-shaped to large multipolar. No clear topographic patterns emerged when the location or distribution of either intrahemispheric or interhemispheric double-labeled cells was compared. It was noted, however, that most double-labeled neurons were situated in zones of overlap that occurred between single-labeled neurons projecting to one or the other of the injected lobules. Conversely, in some situations, sharp borders were maintained between adjacent groups of single-labeled neurons. These studies have demonstrated the existence of both intrahemispheric and interhemispheric branching in the pontocerebellar system. The interhemispheric category included pontine neurons which distributed bilaterally to either homotopic or heterotopic lobules in each hemisphere. The overall distribution pattern of double-labeled pontine neurons, those whose axons distribute to more than one hemispheral lobule, is extremely complex and, although some trends were noted, no system-wide topographic organization was apparent. The category of double-labeled neurons included a wide variety of shapes and sizes ranging from the smallest to the largest types of pontine neurons. Most double-labeled neurons were intermixed with single-labeled pontine cells which projected to one or the other of the two injected lobules.(ABSTRACT TRUNCATED AT 400 WORDS)