The prognostic significance of head-down tilt at -15 degrees and -30 degrees for 6 min at each step was assessed in 40 normal men, aged 22-39. The study of mean values of the maximal amplitude, relative duration of the anacrotic phase, dicrotic and diastolic indices of hemispheric and bimastoidal rheoencephalography as well as of health condition helped to determine cerebral circulation in test subjects with good and moderate tolerance to head-down tilt and to define the criteria of tilt tolerance. The test subjects who showed good tolerance to head-down tilt displayed a simultaneous compensatory increase of the tone of large caliber arteries and arterioles that was accompanied by a noticeable increase in cerebral pulse blood filling at -15 degrees and a moderate increased at -30 degrees as well as the feeling of a slight blood rush to the head. The test subjects who displayed moderate tolerance to head-down tilt showed only an increase in the tone of large caliber vessels; they also exhibited a marked decrease in the tone of arterioles, venules and veins (especially at -30 degrees) which was combined with a significant increase in the cerebral pulse blood filling and an appearance of marked venous waves on rheoencephalograms.