Cerebral venous outflow in rhesus monkeys anesthetized with halothane has been studied by angiographic techniques. The effects of body position and pressure within the airway have been studied. With the monkey supine, injection of radiopaque contrast medium into the superior sagittal sinus permitted visualization of the vertebral plexus and internal jugular veins bilaterally. Pressure on the airway decreased the amount of contrast medium appearing in the vertebral plexus and the superior vena cava and caused engorgement of the internal jugular veins. With the animal erect, almost all contrast medium flowed through the vertebral plexus, with the jugular veins barely visible. With pressure applied to the airway, there appeared to be retrograde flow from the vertebral veins into the internal jugular, subclavian and axillary veins. The vertebral plexus was well outlined as far down as the seventh thoracic vertebra and the density of the upper cervical vertebrae increased. These studies demonstrate that the vertebral venous plexus is an important cerebral venous outflow tract in the supine position, and in the erect position it appears to be the major pathway of exit of cerebral blood.