An apparatus for successful repetitive, indirect photokymographic recordings of systolic blood pressure (Ps) on a human centrifuge during World War II is described. Proportionately to 1 Gz hydrostatic heart-to-head Ps differences, eye level Ps decreased approximately 30 mm Hg per delta G, during 15 s Gz exposures of three relaxed volunteers, a finding confirmed subsequently by continuous intra-arterial pressures in 18 subjects. At 2 G.s-1 onset rates, maximal decreases occurred within 7 s after 1.5 Gz followed by compensatory recoveries while acceleration was maintained. Contours of simultaneously recorded decreases in ear opacity (blood content) were closely similar. Anti-G suit protection against subjective visual symptoms was similar to objective decreases in ear blood content but significantly less than against decreases in Ps at eye level. This may be due to relatively higher intraocular pressures secondary to increased intra-abdominal, cerebrospinal fluid and venous pressure produced by inflation, particularly of the abdominal bladder of the suit.