In a pa-tient with a demonstrated intracerebral vascular tumor, the resting internal jugular venous oxygen saturation and hyper-] ventilation caused the O2 saturation of internal jugular venous blood to approach arterial levels (95%). Since, normally, hyperventilation causes the O2 saturation of internal jugular to fall, in a minority of instances, or to rise only slightly (highest O2 satn. in 69 control cases being 76%), the finding of internal jugular venous O2 saturations which approach those of arterial blood during hyperventilation is of significance in the diagnosis of intracerebral vascular tumors.