In a previous study the collateralization of the rubrospinal tract in the spinal cord of rat, cat and monkey was studied by means of the fluorescent retrograde double labeling technique. In the present study the existence of rubrospinal collaterals to the cerebellar interpositus nucleus (NI) has been studied using the same technique. In rat 'True Blue' (TB) was injected in the cerebellar NI and 'Nuclear Yellow' (NY) was injected ipsilaterally in white and gray matter of C5-C8 spinal segments. In some cases a new fluorescent retrograde tracer was used instead of NY, i.e. 'Diamidino Yellow' (DY), which produces retrograde labeling similar to NY but which migrates only very slowly out of the retrogradely labeled neurons. In these experiments only very few single TB-labeled rubrocerebellar neurons occurred, but many (+/- 90%) of the TB-fluorescent rubrocerebellar neurons were TB-NY or TB-DY double-labeled from the spinal cord. At least 37% of the NY and DY-fluorescent rubrospinal neurons were NY-TB and DY-TB double-labeled from the cerebellum. These findings indicate that, in rat, almost all rubrocerebellar fibers represent collaterals of rubrospinal neurons, and that at least 37% of the rubrospinal neurons give rise to such cerebellar collaterals.