Abstract: In order to prove that the veinous drop of Mannitol makes BBB open. We made the contral on 4 groups of rabbits: normal intracranial pressure group for injecting mannitol and for injecting no Mannitol; the intracranial high pressure group for injectiou Mannital andfor injecting no Mannitol. It revealed that the veinous drop of 20％ of Mannitol at 10ml/kg#183min in the intracranial high pressure make BBB open, for normal intracranial pressare so did for injecting Mannital didn't make BBB open.