Cross-sectional internal diameter measurements were made of right and left common carotid and right femoral arteries and right and left internal jugular, superficial femoral, and common femoral veins in 32 normal human subjects utilizing duplex ultrasonography. The relationships of these vessel sizes to the subject's sex, age, height, weight, and body surface area were analyzed statistically; and graphs were constructed, indicating the relationship of blood vessel diameters to the various body size parameters. Findings indicate that 1) for the femoral veins, body surface area had the best correlation with the internal diameter of the vein; 2) for the right internal jugular vein, body weight had the best correlation with the internal diameter of the vein; 3) correlation between vein diameter and body size of the subject is better for the femoral veins than for the internal jugular veins; 4) internal diameter of the femoral and internal jugular veins increases about 20% when they are distended by 15 degrees of positional inclination of the subject's body; 5) neither age nor sex of the subject influences the positional distensibility of the veins examined; 6) the cross-sectional internal diameter of the femoral and internal jugular veins, as determined by duplex ultrasonography, closely relates to the external diameter of these vessels as measured by direct in vivo application of calipers and to the maximum outside diameter of cannula the vessel will accept.