To minimize hemolysis rate and damage to blood due to characteristics of aortic cannulae, critical blood velocity, blood cannula interactions and shear stress ranges for extracorporeal circulation (ECC) must be determined. At blood flow rates and viscosity levels typically experienced during ECC mean blood velocity, critical blood velocity, shear stress, and kinetic energy are determined in several aortic cannulae, 3-8 mm. Twenty-five percent glycerol is the priming solution. In ten Fr and twenty-four Fr aortic cannulae at 25°C, the velocity above which flow becomes turbulent occurs at flow rates of 975 ccjmin and 2750 ccjmin respectively. Turbulence within these cannulae occurs at flow rates above 650 ccjmin and 2000 ccjmin at 37°C. The recommended flow rate for the cannula sizes tested, determined by the average flow rate at each limit studied, resulted in maximum recommended flows of 883, 1283,2134,2899,3840, and 4954 ccjmin for cannulae sizes 3-8 mm respectively. Blood velocity and shear stress at typical ECC flow rates caused significant turbulence and hemolysis, which must be minimized by the choice of cannulae of the correct size.