There are two major problems here that are not independent. One is the more practically oriented problem of determining the effect of various modes of exercise training on gravitational tolerances, i.e., the point of syncope (unconsciousness) usually estimated from the time of appearance of presyncopal signs and symptoms. The other is more theoretical and concerns the mechanism of blood pressure failure that results in syncope. In many experimental designs these two problems or purposes have been intermingled, with equivocal results.