The hypothalamic projection to the cerebellar nuclei and cortex in the cat was studied by means of retrograde transport of wheat germ agglutinin-horseradish peroxidase complex and various fluorescent tracers. The hypothalamocerebellar nuclear projection originates from various parts of the posterior hypothalamus and reaches mainly the ipsilateral fastigial and interposed nuclei, but all nuclei receive some hypothalamocerebellar fibres. It appears from our double labelling experiments that at least one half of the hypothalamocerebellar nuclear neurones by means of axon collaterals also projects to the cerebellar cortex. Experiments with depositions of fluorescent tracers in both cerebellar hemispheres show that some hypothalamocerebellar fibres branch to reach different parts of the cerebellar cortex. Previous studies have shown that hypothalamocerebellar axons may be branches of hypothalamic efferents to other sites. However, experiments with combined fluorescent tracer depositions in the cerebellum and hippocampus gave no evidence for hypothalamic neurones with axon collaterals to both these regions.