In 30 patients with intractable generalized epilepsy treated with cerebral commissurotomy, the corpus callosum was stimulated intraoperatively at a 1 cm interval with electric current, and evoked potentials (EPs) were recorded from different areas of the brain for determining the distribution patterns of functional projections from the corpus callosum to the cerebral cortex. The surface of the corpus callosum in man was 12 cm long and it was divided into 12 segments (1 cm each). Stimulation of the first segment resulted in EPs only in the frontal lobe. Stimulation of segments 2-4 produced EPs mainly in the anterior, middle and posterior frontal lobe, anterior and central temporal lobe, rarely in parietal and occipital lobes. Stimulation of segments 5-8 induced EPs mainly in the frontal and temporal lobes, but rarely in the parietal and occipital regions; on stimulation of segments 9-11, EPs occurred only in the parietal and occipital regions. These results were confirmed electrophysiologically in 5 cats and anatomically in 8 cats. These data provide an anatomical basis for selective cerebral commissurotomy in the treatment of intractable epilepsy. Hence in patients with a concentration of epileptic discharges in the frontal lobe, the 4 anterior segments of the corpus callosum should be incised. Epileptic discharges in the frontotemporal region indicates that the middle segments should be incised and epileptic activities originating from the parieto-occipital regions can be treated effectively by selective section of the posterior corpus callosum (segments 9-12).