The 3rd ventricle is one of the most surgically inaccessible areas in the brain. It is impossible to reach its cavity without incising some neural structures. Twenty-five cadaveric brains were examined in detail to evaluate the surgically important relationships of the walls of the 3rd ventricle. The routes through which the 3rd ventricle can be reached are: (a) from above, through the foramen of Monro and the roof after entering the lateral ventricle through the corpus callosum or the cerebral cortex; (b) from anterior, through the lamina terminalis; (c) from below, through the floor if it has been stretched by tumor; and (d) from posterior, through the pineal region or from the posterior part of the lateral ventricle through the crus of the fornix. The posterior part of the circle of Willis and the basilar artery are intimately related to the floor, the anterior part of the circle of Willis and the anterior cerebral and anterior communicating arteries are related to the anterior wall, and the posterior cerebral artery supplies the posterior wall. The deep cerebral venous system is intimately related to the 3rd ventricle; the internal cerebral vein is related to the roof, and the basal vein is related to the floor. The junction of these veins with the great vein forms a formidable obstacle to the operative approach to the pineal gland and the posterior part of the 3rd ventricle.