Objective: To study the biologic effects of various concentrations of thrombin on aquaporin 4 (AQP4) expression in rat primary cultured astrocytes, and to explore the regulation mechanism of transmembrane water transportation in astrocytes after intracerebral hemorrhage (ICH). Methods: Primary cultured astrocytes were incubated in culture mediums containing various concentrations of thrombin for 24 h and harvested. AQP4 mRNA and AQP4 protein expression were determined by reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemical technique. Cell apoptosis was detected by TdT-mediated dUTP nick end labeling (TUNEL) technique. Cell morphology was observed by phase contrast microscope, and cell viability was assayed by MTT. Results: AQP4 mRNA and AQP4 protein showed a low expression in normal astrocytes. The expression of AQP4 mRNA and AQP4 protein significantly increased in the astrocytes treated with 100 U/ml or 200 U/ml thrombin (P ＜ 0.01),and these astrocytes swelled. The number of TUNEL positive cells significantly increased. On the other hand, AQP4 mRNA and AQP4 protein expression were down-regulated in the astrocytes treated with 0.5 U/ml or l U/ml thrombin (P ＜ 0.05),and the cell morphology did not change. Few TUNEL positive cells were observed. Conclusion: AQP4 over-expression induced by high concentrations of thrombin causes an increased permeability of water in astrocytic membrane. On the contrary, the decreased AQP4 expression prevents the astrocytes from swelling and apoptosis.