Spatial localization and axonal branching in mossy fiber projections to two rostrocaudally-separated regions of the 'forelimb' c1 zone in lobulus simplex and paramedian lobule were studied in rats using a retrograde double-labelling tracer technique. In four animals the two cortical regions were localized electrophysiologically and each was micro-injected with tracer material, yielding a total of eight different cases. Single- and double-labelled cell bodies were plotted in the basal pontine nucleus (BPN), nucleus reticularis tegmenti pontis (NRTP), and the lateral reticular nucleus (LRN). As a control, cells labelled in the contralateral inferior olive were also counted. The parts of the c1 zone in lobulus simplex and the paramedian lobule were found to receive mossy fiber inputs from similar regions of BPN, NRTP and LRN. Double-labelled cells were not found in NRTP but were present in BPN and LRN (on average 6% and 25% of the smaller single-labelled population, respectively). The incidence of double-labelled cells in the olive and LRN was positively correlated, but no relation was found between olive and BPN, suggesting a zonal organization within the mossy fiber projections from LRN, but not from the pons. In quantitative terms, the c1 zone in lobulus simplex received a greater density of mossy fiber projections from BPN, NRTP and LRN than the c1 zone in the paramedian lobule. This suggests that the two parts of the same cerebellar cortical zone differ, at least partially, in regard to their inputs from three major sources of mossy fibers. This is consistent with the modular hypothesis and could enable a higher degree of parallel processing and integration of information within different parts of the same zone.