Hemodynamic responses to combined heavy dynamic leg exercise (hiP), breath holding (BH) and gravity-induced blood volume shifts direction were studied. Thirteen subjects were studied at normal gravity and 12 during parabolic flight, performing 20 s hiP or combined hiP&BH (stimulus period) from a baseline of 30 W at normal gravity (1 G ). Heart rate and mean arterial pressure responses to BH were similar between gravity conditions, but stroke volume (SV) differed markedly between gravity conditions: at 1 G SV was higher [112 ± 16 ml (mean ± SD)] during BH, than during eupnea [101 ± 17 ml (P < 0.05, N = 13)]. In weightlessness the corresponding SV values were 105 ± 16 and 127 ± 20 ml, respectively (P < 0.05, N = 6). Transthoracic electrical conductance (TTC) was used as index for intrathoracic volume. TTC fell significantly during BH. This decrease was attenuated in weightlessness. It is concluded that the transient microgravity temporarily reduces the efficiency of the muscle pump so that the deep inspiration at the onset of the high-intensity exercise and breath-hold period cannot augment venous return as it could during identical manoeuvres at normal gravity. © Springer-Verlag 2009. z+ z+