Present study observed the direct effects of hypoxia on expression of inducible nitric oxide synthetase(iNOS)in endothelial cells(CMEC)which isolated from rats'cerebral cortical microvessels,using reverse transcription polymerase chain reaction(RTPCR)technique. Hypoxic mixed gases were over the cultural endothelial ce11s in different time with BNA310N2-O2-CO2 incubator(TAEAI ESPEC Co).The experimental results showed that serious hypoxia,the partial pressure of O2 is 6.7kPa in cultural medium,can induce the expression of iNOS gene in CMEC after 24 hours,reoxygenation cannot entire eliminate the effect.The moderate hypoxia, the partial pressure of O2 is 10.7 kPa, cannot induce the expression of iNOS gene,These results suggested that hypoxia could be an inducer of production of iNOS in CMEC and NO may play an importent role in regulation of cerebral microcirculation under hypoxia.