The anatomical foundations for the endoscopic fenestration of the septum pellucidum have not been presented previously in an isolated fashion. Using a surgical microscope, the microsurgical anatomy of the septum pellucidum, lateral ventricles, and surrounding structures was examined in ten adult brains injected with silicone rubber. The mean anteroposterior length of the septum pellucidum was 41 mm (range 40-42). Its mean height was 13.11 mm (range 12-15) at the level of the foramen of Monro; 10 mm (range 9-11) at the frontal horns, and 8 mm (range 7-10) at the ventricular atrium. One to three septal veins delimited 2 or 3 avascular areas in the septum pellucidum. We found an anterior area which was consistently avascular and large enough to be fenestrated safety. More posterior areas, while avascular, were inconsistent in their size and shape.