To investigate the expression of NOS Ⅲ mRNA and protein in cultured porcine cerebral arterial endothelial cells (CAEC) during hypoxia and reoxygenation and the effects of L-Tetrahydropalmatine (L-THP) on the gene expression of NOS Ⅲ in CAEC during hypoxia and reoxygenation. The cultured CAEC were divided into 5 groups: control, hypoxia, hypoxia+ reoxygenation, hypoxia+L-THP and reoxygenation+L-THP groups. NOS Ⅲ mRNA expression was detected by reverse transcription-polymerase chain reaction (RT-PCR). Immunocytochemistry was used to detect the level of NOS Ⅲ protein. The expression of NOS Ⅲ mRNA and protein were increased when CAEC were exposed to hypoxia for 1 h, and significantly decreased during reoxygenation 2, 6 and 12 h after 1-h of hypoxia. L-THP from 10-8 mol/L to 10-3 mol/L could inhibit the up-regulation of NOS Ⅲ gene expression during hypoxia and down-regulation of NOS Ⅲ gene expression during reoxygenation.