We performed tidal-volume single breath washins (SBW) using tracers of different diffusivity and varied the time spent in microgravity(µG) before the start of the tests to look for time-dependent effects. SF₆ and CH₄ phase III slopes decreased by 35 and 26 % respectively in µG compared to 1G (P<0.05) and the slope difference between gases disappeared. There was no effect of time in µG, suggesting that neither the hyper-gravity period preceding µG, nor the time spent in µG, affected gas mixing at volumes near FRC. In previous studies using SF₆ and He (J. Appl. Physiol. 82: 859-865, 1997), the vital-capacity SBW showed an increase in slope difference between gases in transient µG, the opposite of the decrease in sustained µG. In contrast, tidal-volume SBW showed a decrease in slope difference in both µG conditions. Since it is only the behavior of the more diffusive gas that differed between maneuvers and µG conditions, we speculate that in the previous vital capacity SBW, the hyper-gravity period preceding the test in transient µG provoked conformational changes at low lung volumes near the acinar entrance.