Cardiovascular and hormonal responses to +700 head-up tilt (HUT: orthostatic challenge) were compared between six endurance exercise trained (ET) and six untrained (UT) subjects prior to and immediately following 4 hours of -60 head-down rest (HDR). The ET subjects showed a significant (P < 0.05) decrease in orthostatic tolerance time (pre syncopal symptoms) during post-HDR HUT, while no difference was observed between ET and UT groups in pre-HDR HUT. The volume regulatory hormonal responses were similar between ET and UT groups whether during HUT or HDR. The pre-syncopal subjects had a greater increase in plasma arginine vasopressin and less increase in plasma renin activity and plasma aldosterone during HUT than was observed in non syncopal subjects. These data suggest that HDR deconditioning was more effective in the ET subjects.