The fastigiocerebral projection in the cat was investigated electrophysiologically by recording field potentials and unit activities and also morphologically by anterograde and retrograde HRP methods. Three cortical areas mostly hidden in sulci, two in the frontal cortex and one in the insular cortex, were responsive to fastigial stimulation under pentobarbital anesthesia. The responsive areas in the frontal cortex were the ventral bank of the cruciate sulcus and the area surrounding the fundus of the presylvian sulcus; the latter area corresponds to a subregion of the frontal eye field. The responsive area in the insular cortex was the ventral bank of the anterior ectosylvian sulcus, which overlaps largely with the "anterior ectosylvian visual area." The response in the frontal cortex was a surface-positive, depth-negative wave, whereas the response in the insular cortex was a surface-negative, depth-positive wave. Anterogradely labeled terminals of the fastigiothalamic projection were most dense in the ventromedial (VM) nucleus in which retrogradely labeled neurons were numerous when WGA-HRP was injected into any one of the three cortical areas. In agreement with the results of the HRP studies, units that responded orthodromically to fastigial stimulation and antidromically to cortical stimulation were located in the thalamic VM nucleus. There was a marked difference between the frontal and insular cortices in laminar distribution of terminals of the thalamocortical projection fibers. Anterogradely labeled terminals after injection of WGA-HRP into the VM nucleus were distributed mainly in layers I and III in the frontal cortex, whereas they were distributed mainly in layer I in the insular cortex.