If the sigmoid and transverse sinuses could be divided safely, surgeons could broaden their access during skull base surgery without retracting the brain extensively. We undertook this study in monkeys to assess the risk of sacrificing these sinuses. We learned that when bilateral transverse sinuses were occluded, the sinus pressure measured in the superior sagittal sinus increased significantly as blood was purged from sinus venules. Unilateral sacrifice of sigmoid and transverse sinuses, however, did not change proximal sinus pressure in any ipsilateral or contralateral sinus. Furthermore, no change in regional cerebral blood flow or motor evoked potentials was seen in the parietal lobe nor did auditory brainstem response change significantly with sinus ablation. We conclude that, unilaterally, sigmoid and transverse sinuses may be ablated safely because collateral circulation via the vein of Labbé, superior petrosal sinus, petrosal vein, and backflow from the transverse sinus to the contralateral sinus may be established.