During ten operations for spinal dural arterio-venous fistulas (AVF), intraoperative measurement of flow velocity and intravascular pressure was performed. Flow velocities were recorded using a miniaturized Doppler probe. Intravascular pressure changes in the draining veins before and after AVF removal was measured with small needles. Varying the pCO2 between 20 mm Hg and 60 mm Hg, the flow velocities in arteries supplying the spinal cord were investigated so that the vasomotoric reactivity of the peripheral vessel wall in cord tissue was investigated before and after AVF occlusion. The flow velocities in dural AVF feeders were not as high as those known from cerebral angioma feeders. In addition, they often showed lowered end-diastolic flow velocity as a sign of increased vascular resistance, thus proving impaired venous outflow from the spinal canal. After excision of the local fistula, the vessels supplying and draining the spinal cord showed improved circulation. In the former recipient veins, no further flow could be recorded. The venous pressure in dural AVF was about 70% of the systemic arterial pressure. Fistulas presenting a high shunt volume on angiography showed only moderately increased venous pressure and a more pronounced pressure drop after fistula occlusion as compared with low-volume fistulas. The CO2 reactivity of vessels supplying the spinal cord was normal before and after AFV removal.