Pilots flying high-performance aircraft are commonly exposed to sustained inertial forces in the head-to-foot direction corresponding to up to 9 times the pull of Earths gravity (9 G). Under such conditions, arterial pressure at heart level must be elevated to 250-300 mmHg to overcome the exaggerated hydrostatic pressure gradients that act along the arteries from the heart to the head, and hence to maintain blood perfusion of the brain. There is a considerable interindividual variability as regards the capacity to withstand high sustained G-loads (G-tolerance) in the relaxed state. Relaxed G-tolerance improves substantially in response to repeated exposures to high G-loads. It appears that such G-habituation is brought about by increased arterial baroreflex sensitivity as well as reduced distensibility of dependent blood vessels. Though the mechanisms underlying these adaptive changes are not fully understood it seems that the repeated exposure to increased local transmural pressure constitutes the stimulus for the G-training induced increase of the wall stiffness of dependent arteries, precapillary resistance vessels and veins. Finally, it should be emphasized that autonomic regulation of arterial pressure is not sufficient to meet the demands at very high sustained G-loads. Under such conditions arterial pressure must be increased by means of voluntary muscular straining maneuvers, and by support systems in terms of anti-G suits and positive pressure breathing.