In heterozygous (Jj) and homozygous Gunn rats (jj), cerebellar noradrenergic innervation was examined using immunohistochemical, neurochemical and electrophysiological techniques. Immunohistochemical analysis using an antiserum against tyrosine hydroxylase (TH) revealed a marked enhancement in immunoreactivity largely in the granular layer and the whole nuclei in the jj cerebellum, resulting from an increase in TH-immunoreactive varicose fibers forming synapse-like structures on the somata and dendrites of granule cells or nuclear neurons. The concentration of norepinephrine in both the cortical and nuclear regions of the jj cerebellum was significantly higher than that in the control, whereas no significant difference of this total amount was observed between the jj and Jj cerebella. Injection of norepinephrine into the Jj cerebellar nuclei reduced the firing rate of spontaneous unitary discharges of neurons in the interpositus nucleus. These findings suggest that the the jj cerebellum causes an enhancement of the noradrenergic innervation which may possibly be one of its characteristic alterations.