There is ample evidence that posture affects many cardiorespiratory variables, but the extent to which secondary reflex mechanisms complement or oppose the primary gravity effect is not clear. We have addressed ourselves to this problem by studying five normal volunteers, passively tilted from the supine to the upright position in 15 degrees increments, in random sequence, determinging cardiac output (Q), heart rate (HR), stroke volume (SV), minute and alveolar ventilation (VE and VA), functional residual capacity (FRC), and arterial-end-tidal PCO2 pressure difference. In each position, four to five measurements were obtained by noninvasive techniques. Changes in Q and in FRC were linearly related to the sine of the tilt angle, indicating that reflexes were either absent or that their net effect was proportional to the effects of gravity; this was clearly not the case for other variables (HR, SV, VE, VA) in which it was possible to demonstrate threshold values for the appearance of secondary changes.