Vascular and neoplastic lesions of the lateral and III ventricles, the pineal region, or the basal ganglia affect highly critical areas. For surgical treatment of these lesions, the interhemispheric, transcallosal approach was studied in our laboratory in an attempt to evaluate the surgical procedure. The advantages and disadvantages are described in comparison with those for other approaches in the same regions. To this end, we dissected 20 anatomical specimens interhemispherically using the surgical microscope (with a magnification of 6 x to 25 x); the specimens were fixed for 6-8 weeks in formaldehyde 10%). In each specimen the corpus callosum was exposed and then sectioned to a length of 2 to 2.5 cm over its anterior, middle, and posterior thirds. The anatomy of the lateral and III ventricles and of the pineal region was explored; the arrangement configuration of the anterior cerebral arteries and their branches was also evaluated, as were the thalamostriate and other subependymal veins, the internal cerebral veins, the choroid plexus, the trigone, and the septum pellucidum.