To determine the extent of the role that gravity plays in dispersion and deposition during breath holds, we performed aerosol bolus inhalations of 1-µm-diameter particles followed by breath holds of various lengths on four subjects on the ground (1G) and during short periods of microgravity (µG). Boluses of ~70 ml were inhaled to penetration volumes (V_p) of 150 and 500 ml, at a constant flow rate of ~0.45 l/s. Aerosol concentration and flow rate were continuously measured at the mouth. Aerosol deposition and dispersion were calculated from these data. Deposition was independent of breath-hold time at both V_p in µG, whereas, in 1G, deposition increased with increasing breath hold time. At V_p = 150 ml, dispersion was similar at both gravity levels and increased with breath hold time. At V_p = 500 ml, dispersion in 1G was always significantly higher than in µG. The data provide direct evidence that gravitational sedimentation is the main mechanism of deposition and dispersion during breath holds. The data also suggest that cardiogenic mixing and turbulent mixing contribute to deposition and dispersion at shallow V_p.