Abstract： Objective To explore the possible pathogenesis of chronic posthemorrhagic hydrocephalus by observing the expression of AQP1 on rats after experimental intraventricular hemorrhage. Methods Fourty-five rats were randomly divided into normal control(n=5),sham-operated(n=20)and experimental(n=20)groups.The 0.1mL saline and 0.1mL citrated autologous blood were injected into the lateral ventricle of the rats in the sham-operated and the experimental groups,respectively.The later two groups were divided into four subgroups according to different time points at 3,7,14 and 30 d(n=5).The change of protein expression of AQP1 at different time points of bleeding were detected by immunohistochemical techniques and that of mRNA expression of AQP1 was obsevred by in situ hybridization,respectively.and then,the possible pathogenesis of chronic posthemorrhagic hydrocephalus was discussed. Results At the time point of 30 d after intraventricular hemordmge,chronic hydrocephalus appeared in 4 rats(80%)in the experimental group.High protein expression of AQP1 was found in the apical of cuboidal epithelium of choroids plexus,the ependyma, the pia mater, the arachnoid and the dura in the normal control group;the protein expression of AQP1 gradually weakened in the experimental group 3 d aftea intraventricular hemorrhage and dropped to the bottom on the 14th d,which was significantly different from the normal control and sham-operative groups(P<0.05).The mRNA expression of AQP1 was weaker than the protein expression of AQP1,and the expression locations of them were basically in concordance. Conclusion AQP1 is involved not only in the secretion of cerebrospinal fluid,but also in the process of the CSF absorption.The decrease of CSF absorption induced by the decreased expression of AQP1 after intraventrieular hemorrhage in rats may also relate to the development of chronic posthemorrhagic hydrocephalus.