Some physiological responses to head-up tilt and 3 h standing were evaluated in 13 dehydrated subjects. Seven of the subjects proved to be orthostatically intolerant (INT), exhibiting presyncopal symptoms. Before the symptoms manifest themselves the INT subjects had consistently lower (p less than 0.05) systolic blood pressures, generally lower diastolic and pulse pressures, and elevated (p less than 0.05) plasma renin activity (PRA) compared to the tolerant (TOL) subjects. Plasma vasopressin usually increased more in the INT subjects, but appeared to be related to the severity of presyncopal symptoms rather than to the upright posture per se. It is concluded that systolic and pulse pressures, with PRA, may allow discrimination between TOL, and potentially INT individuals; i.e., predict orthostatic intolerance. It is suggested that dehydration could provide a valuable physiological model for elucidating the causes of orthostatic intolerance.