Studies were conducted on twelve rabbits which were lesioned in various regions of the cortex and cerebellar nuclei. The survival time, in ten animals, was found 4 to 10 days and in one animal 375 days. In the twelfth rabbit further lesions were made in the fastigial nucleus, 380 days after multiple lesions of the posterior cerebellar region. Survival time, in this case, was 9 days. Frozen sections of the cerebellum and brain stem were impregnated according to the Nauta-Gygax modified method (NAUTA 1957, Fink-Heimer 1967, Loewy 1969) and the course and termination of the nerve fibres into the inferior colliculus of the mesencephalic tectum has been studied. Degenerative fibres in this region of the brain stem were seen only after lesion of the fastigial nucleus or of its efferent fibres. The nucleus fastigii project the fibres to the inferior colliculus nuclei on both sides in the ascending ramus of the uncinatus fasciculus. These fibres reach the inferior colliculus nuclei mainly on the contralateral side and, to a lesser degree, on the same side. Results of the studies are discussed in the light of the latest neurophysiological researches.