We hypothesized that the perfusion heterogeneity in the human, upright lung is determined by non-gravitational more than gravitational factors. Twelve and six subjects were studied during two series of parabolic flights. We used cardiogenic oscillations of O₂ / SF₆ (hexafluoride) (COS O₂ / SF₆) as an indirect estimate of intra-regional perfusion heterogeneity (series 1) and phase IV amplitude (P₄) as a indirect estimate of inter-regional perfusion heterogeneity (series 2). A rebreathing - breath holding - expiration maneuver was performed. Inflight, breath holding and expiration were performed either in microgravity (0 G) or in hypergravity. Controls were performed at normal gravity (1 G). In series 1, expiration was performed at 0 G. COS O₂ / SF₆ was 19% lower when breath holding was performed at 0 G than when breath holding was performed at 1 G (means ± SD, 1.7 ± 0.3 and 2.3 ± 0.6 % units) (P = 0.044). When breath holding was performed at 1.8 G, values did not differ from 1 G control (2.6[plusmn]] 0.8 % units, P = 0.15) but they were 17% larger at 1.8 G than at 1 G. In series 2, expiration was performed at 1.7 G. P₄ changed with gravity (P < 0.001). When breath-holding was performed at 0 G, P₄ values were 45 ± 46% of control. When breath-holding was performed at 1.7 G, P₄ values were 183 ± 101% of control. We conclude that more than half of indices of perfusion heterogeneity at 1 G are caused by non-gravitational mechanisms.