Acceleration (+Gz) handicaps gas exchange in man by creating a ventilation/perfusion (VA/Q) mismatch in the lung. At 1 G, leg exercise makes VA/Q more nearly homogeneous, primarily because peripheral pumping reduces venous pooling and improves pulmonary capillary filling. Four men were studied at rest and pedaling 0, 300 and 600 kpm/min at +1, 2, and 3 Gz. Acceleration raised the resting Vo2, VE, VE/Vo2 and HR but lowered O2 pulse and PetCO2. Since acceleration increased the cost of moving the legs (unloaded pedaling), all work data were considered as a function of VO2. Low levels of exercise returned all variables toward their control (1 G) values. At higher workloads, the combination of G with exercise caused divergence from control measurements, probably also reducing maximal work capacity. Cardiogenic oscillation amplitude increased with G and decreased with work, evidence that exercise improved VA/Q homogeneity at all G-levels. Where man must undergo prolonged exposure to moderate acceleration, use of the peripheral pump might offer some advantages over the conventional anti-G suit.