A few of flow experiments were utilized to verify a theoretical hypothesis proposed by Fung and coworker which showed that the tensile stress in the upper cell membrane of the vascular endothelium could accumulate upstream against the direction of blood flow. Endothelial cells from replicate human umbilical vein segment (HUVSEC) in vitro with length of 11 cm and 21 cm were exposed to the same pulsatile laminar shear stress averaged of 0.12 N/m2 for 42 hours. The average production rate of endothelin-1(ET-1), at 11 cm segment is 50% lower than that at 21 cm segment(16.93 +/- 0.89) vs. (26.13 +/- 1.79) pg/cm2.h respectively. The average production rate of ET-1 under pulsatile laminar flow was significantly higher than that under steady laminar flow. It showed that, high correlation of the length of HUVSEC with their ET-1 metabolism exists, suggesting that the tensile stress in the upper endothelial cell membrane could accumulate.