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Many workers have shown that the cardiac output in normal man is less in the upright than in the recumbent position (McMichael and SharpeySchafer, 1944;Stead et al., 1945;Coe, Best, and Lawson, 1950;Wang, Marshall, and Shepherd, 1960;Chapman, Fisher, and Sproule, 1960;Reeves et al., 1961). The physiological events underlying this change are complex and depend on many variables, such as the angle of tilt and length of time the patient remains at a given angle and whether the change is from recumbency to an upright position or vice versa. Posture and its relation to cardiac output here becomes increasingly important with the introduction of hypotensive drugs. The purpose of this study was to determine the relation ofthe angle oftilting to the changes in cardiac output, blood pressure, and pulse rate in normal man. SUBJECTS AND METHODS Studies were carried out on 33 subjects: 28 women and 5 men, whose average age was 37 years.The observations were all made three hours after breakfast or lunch. The relative cardiac outputs were measured by the indicator dilution method from dye curves recorded by a photoelectric earpiece (Gabe, Tuckman, and Shillingford, 1962), chopper amplifier, and direct writer recording system. All subjects rested quietly in the supine position for 30 minutes before the first cardiac output determination. Coomassie blue dye was injected from a calibrated syringe through a three-way tap and eighteen-gauge needle into an antecubital vein, a 5 per cent dextrose-in-water drip keeping the system open between injections. In any one subject the injections of dye were of equal quantity and varied from 20 to 30 mg./injection in different subjects.

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