A group of 6 subjects (25-45 y.o.) participated in the studies of the tolerance of lateral (chest-back, +Gx) and longitudinal (head-pelvis, +Gz) loads before and after pharmacological hypohydration. Moderate hypohydration (diminution of the circulating plasma volume by 14% and body mass by 2% on the average) induced by diuretics does not lead to a substantial loss in tolerance of +Gx up to 8.3 units but reduces the threshold of +Gz tolerance by 0.6 units. The scientific and applied sense of these data is that diuretics can be safely administered in the pre-launch period to prevent the negative effects associated with the beginning of space flight.