1. Investigations on stance regulation have already suggested that the body's center of mass is the variable controlled by the CNS to maintain equilibrium. The aim of this study was to determine how the center of mass of the body is regulated when leg movements are made under different gravitoinertial force conditions. 2. Kinematic and electromyographic (EMG) recordings were made during both straight-and-level flight (earth-normal gravity condition, nG) and periods of weightlessness in parabolic flight (microgravity condition, microG). The standing subjects were restrained to the floor (kept from floating away in microG) and were instructed to raise one leg laterally to an angle of 45 degrees as fast as possible. 3. Two modes of center of mass (CM) control were identified during leg movement in nG: a "shift mode" and a "stabilization mode." The shift mode served to transfer the CM toward the supporting side before the leg raising, and it preceded the phase of single limb support. The stabilization mode took place after the CM shift was completed and was aimed at stabilizing the CM during raising of the leg. In this phase, the movement of the raising leg is counterbalanced by a lateral inclination of the trunk in the opposite direction. As a consequence, CM position did not change with respect to the position reached before the leg raising, and its projection on the ground remained within the support area delineated by the stance foot. 4. Under microG, the CM position did not change before the leg raising. Moreover, gastrocnemius medialis activity observed in the moving leg under nG, preceding the initiation of the body weight transfer toward the supporting leg, was greatly reduced. While the leg is raising, the simultaneous and opposite lateral trunk movement was still present in microG. 5. Results suggest that the body weight transfer corresponding to the shift mode, might depend on the gravity constraints, whereas the stabilization mode, which remains unchanged in microG, might be a motor stereotype that does not depend on the gravity conditions.