Background. At high +Gz-loads, pressures in dependent blood vessels are commonly raised to levels capable of distending arteries and arterioles. Distension of precapillary vessels may reduce arterial pressure, and hence compromise G-tolerance. Methods. Pressure distension in arteries and arterioles of the leg were compared in subjects possessing high G-tolerance (H; n=8; relaxed Gradual Onset Rate G-tolerance = 6.6±0.7 G) and low G-tolerance (L; n=8; G-tolerance=3.9±0.3 G). The two groups were closely matched with regard to age, weight, height and resting arterial pressure. Experiments were performed with the subject supine in a pressure chamber with a lower leg protruding through a port in the chamber door. Increased intravascular pressure in the blood vessels of the leg was accomplished by increasing chamber pressure in a stepwise manner to a maximum of 240 mmHg. Diameter and flow in the posterior tibial artery were measured by means of ultrasonographic/Doppler techniques. Results. Arterial stiffness, as assessed by the critical distending pressure (i.e. the local arterial pressure at which a marked sustained increase in arterial diameter was initially detected), was higher (p=0.01) in the H-group (316±27 mmHg) than the L-group (281±21 mmHg). The critical arteriolar distending pressure (i.e. the local arterial pressure at which a marked sustained increase in flow was initially detected) tended to be higher (p=0.07) in the H-group (282±15 mmHg) than the L-group (264±11 mmHg). Conclusion. It appears that the in vivo wall stiffness of precapillary leg vessels may be a determinant for relaxed G-tolerance.