In 28 dogs with controlled ventilation and circulation, the passage of microspheres (MS) injected into the carotid in size ranges from 1μ to 100μ through the brain was observed. A cannula placed into a cortical vein provided recovery of MS from pure cortical venous blood and a semicontinuous outflow measurement. In control animals MS up to 7μ passed freely through the brain; bigger MS were almost completely entrapped. In states of ischemic and hypoxemic hypoxia, as well as in prolonged reactive hyperperfusion caused by a previous hypoxic exposure, MS up to 76μ passed through the brain within two minutes after injection. Hypercapnia did not change the filter capacity. The presence in the dog's brain of arteriovenous connections much larger than capillaries, but not larger than 76μ, is postulated. These connections, closed in the normal brain, are opened by hypoxia.