The frequent use of arterial puncture, cannulation, and A-V shunting has demonstrated a greater degree of safety than could be predicted from previous anatomic and angiographic studies. Using a noninvasive technique, the status of the superficial palmar arch (SPA) and relative contributions of radial and ulnar arteries were determined in 100 volunteers with no history of vascular disease. Although there was no significant difference in the diameter of the vessels at the wrist, the flows showed statistically significant ulnar dominance, suggesting that the difference is a factor of a lower distal resistance on the ulnar side. The lower resistance across the ulnar bed probably is responsible for the clinically observed ease of sacrifice of the radial artery contribution to palmar flow. The SPA was found to be incomplete in 11.0% of the hands.