Projection of inferior olive (IO) neurons to the deep cerebellar nuclei (CN) was investigated in the rat by reconstructing single axons that were labeled with biotinylated dextran amine injected into the IO. All reconstructed terminal arborizations in the CN (n = 18) arose as collaterals from climbing fibers (CFs). One to six nuclear collaterals were given off from each of six CFs that were reconstructed along the nearly entire pathway backward from cortical terminal arborizations to the IO. Nuclear collaterals were much thinner (0.2-0.3 micron in diameter) than stem axons projecting to Purkinje cells (0.7-1.4 microns). The number of swellings per a single nuclear collateral ranged from 24 to 118 (n = 18). Terminal arborizations of nuclear collateral originating from a single CF spread for some hundreds of micrometers and occupied a localized portion within the CN.