During the cardiac cycle, cardiogenic oscillations of expired gas concentrations are generated (COS_[x]). At the same time, there are heart-synchronous cardiogenic oscillations of airway flow (COS_flow), where inflow occurs during systole. We hypothesized that both phenomena, although primarily generated by the heart beat, would react differently to the cephalad blood shift caused by inflation of an anti-G suit and to changes in gravity. Twelve seated subjects performed a rebreathing - breath-holding - expiration maneuver with a gas mixture containing O₂ and helium (He), at normal (1 G) and moderately increased gravity (2 G); an anti-G suit was inflated to 85 mmHg in each condition. When the anti-G suit was inflated, COS_flow amplitude increased (P = 0.0028) at 1 G to 186% of the control value without inflation (1 G control) and at 2 G to 203% of the control value without inflation (2 G control). In contrast, the amplitude of COS of the concentration of the blood-soluble gas O₂ (COS_[O2/He]), an index of the differences in pulmonary perfusion between lung units, declined to 75% of the 1 G control value and to 74% of the 2 G control value (P = 0.0030). There were no significant changes with gravity of either COS_flow or COS_[O2/He] amplitudes. We conclude that the heart-synchronous mechanical agitation of the lungs, as expressed by COS_flow, is highly dependent on peripheral-to-central blood shifts. In contrast, COS_{[blood-soluble gas]} appears relatively independent of this mechanical agitation and seems to be determined mainly by differences in intra-pulmonary perfusion.