Interpeduncular perforating branches were found in all 69 examined posterior cerebral arteries. They varied in number from 1 to 10, with an average of 2. They arose directly from the posterior cerebral artery (47.8%), from the collateral branches of the posterior cerebral artery (30.3%), or by their own common stems (88.4%). Their extracerebral segments varied from 100 to 750 microns in diameter (average, 321 microns). They gave off collateral branches to the cerebral peduncle, posterior perforated substance, oculomotor nerve, and mammillary bodies. Intracerebral segments had proximal and distal portions. The mean diameter of the former was 254 microns, and of the latter, 227 microns. The perforating arteries were divided into short interpeduncular and long mesencephalic and diencephalic vessels. Anastomoses among them were noticed in four-fifths of the cases. Despite this fact, the perforating arteries must be preserved during operations on aneurysms in the interpeduncular fossa.