We compared the cardiopulmonary physiology of eight subjects exposed to 1, 2, and 3 Gz during immersion (35 degrees C) to the heart level with control dry rides. Immersion should almost cancel the effects of gravity on systemic circulation and should leave the lung alone to gravitational influence. During steady-state breathing we measured ventilation, O2 consumption (VO2), CO2 production, end-tidal PCO2 (PACO2), and heart frequency (fH). Using CO2 rebreathing techniques, we measured cardiac output, functional residual capacity, equivalent lung tissue volume, and mixed venous O2 content, and we calculated arterial PCO2 (PaCO2). As Gz increased, ventilation, fH, and VO2 rose markedly, and PACO2 and PaCO2 decreased greatly in dry ride, but during immersion these variables changed very little in the same direction. Functional residual capacity was lower during immersion and decreased in both the dry and immersed states as Gz increased, probably reflecting closure effects. Cardiac output decreased as Gz increased in dry rides and was elevated and unaffected by Gz during immersion. We conclude that most of the changes we observed during acceleration are due to the effect on the systemic circulation, rather than to the effect on the lung itself.