The cerebellar cortex of adult hens contains a dense plexus of thin varicose nerve fibers which display a formaldehyde-induced green fluorescence. This plexus is not distributed at random in the cortical layers. Within the granular layer the plexus forms a netlike pattern. The fiber branches, which have numerous varicosities, are predominantly oriented in the traverse plane of the folium. In the molecular layer the fluorescent plexus shows some variations in the convex, flat and concave portions of the folia. Many of the fluorescent branches are oriented parallel to the course of the folium. They arise from a T-division of radially oriented axons resembling parallel fibers in Golgi sections. The meshes of the fluorescent plexus in the granular layer measure 10-60 mu. In the molecular layer (top of the folia) there are about 30 fluorescent fibers per 100 mu2. The fluorescent fibers originate from the locus coeruleus and form a rostral and a caudal bundle in the cerebellar peduncle. The mode of distribution of the fluorescent fibers in the cortical layers seems to depend on the organization of the innervated tissue. Light microscopy suggests that the aminergic fibers innervate more than one class of cerebellar neurons.