Sixty-five patients with lesions affecting the third ventricle (54 patients) or the corpus callosum itself (11 patients) underwent partial callosotomy or a circumscribed callosal resection. Before the surgery 20 patients were studied using the battery of cognitive, affective and behavioural tests which was repeated 10 and 100 days after surgery. No disconnection syndrome was over observed after the partial commissurotomy. Transcranial magnetic stimulation over the sensorimotor cortex was performed in 10 patients to determine conduction time of callosal fibres by measuring inhibition of tonic voluntary electromyographic activity in muscle's ipsilateral to the activated hemisphere. It was found that this inhibition was absent in patients with lesions of the trunk of the corpus callosum and present in patients with lesions of the genu or splenium. In addition magnetic resonance imaging measurements of the corpus callosum were performed in 40 normal subjects to establish a classification system for corpus callosal area. The results showed a wide variability of the cross-sectional area of the corpus callosum. The comparison of the shape of the corpus callosum lead to a categorisation according to the presence and location of depressions on its surface.