The static mechanical properties of major branches of the human arteries (common carotid artery, abdominal aorta, femoral artery, and brachial artery) were studied in 39 subjects, aged 6-81 years, using an ultrasonic phase locked echo tracking system that allows continuous transcutaneous measurement of the diameter of the artery. The stiffness indices were calculated from the relation between systemic blood pressure and arterial diameter. With advancing age there was a significant increase in the diameter of all arteries with a reduction in percentage change in diameter. The stiffness index increased with age in all arteries; however, in the brachial and femoral arteries there was considerable variation in the individual values for a given age. The age associated increase in stiffness was statistically significant only in the common carotid artery and the abdominal aorta. Although the mechanical properties of the peripheral arteries were significantly influenced by the measuring environment, the calculated stiffness indices were less vulnerable to these stimuli in the central arteries. These results indicate that the stiffness indices of the peripheral muscular arteries are modified appreciably by vasoactive stimuli and that the mechanical properties of the deeper elastic arteries provide sufficiently reliable information about changes caused by aging and arteriosclerosis. The new ultrasonic method used appears to be suitable for this analysis.