Five normal non-smoking subjects inhaled an aerosol of 99Tcm-DTPA in saline with a mass median aerodynamic diameter of 0.6 micron. The rate of clearance (k) of the inhaled 99Tcm-DTPA from lung to blood was measured using a gamma scintillation camera with computer data acquisition. During a single clearance study the subjects voluntarily breathed close to total lung capacity (TLC) and close to residual volume (RV). They breathed at one volume for 5 min, then at the opposite volume for 5 min and finally reverted to the original volume for 5 min with a 1 min pause between each manoeuvre. The order in which each subject performed these breathing patterns was randomized. Tidal volume, respiratory frequency and end expired volume were measured with a water spirometer. When they breathed close to TLC the rate of clearance increased (k = 4.62 +/- 1.03%/min) compared with breathing close to RV (k = 1.96 +/- 0.5%/min). This effect of changing volume was immediately reversible after adopting each new lung volume. There were no significant changes in tidal volume or respiratory frequency between each 5 min period. There was no significant difference between the clearance increased (k = 4.62 +/- 1.03%/min) compared with breathing close to RV (k = 1.96 +/- 0.5%/min). This effect of changing volume was immediately reversible after adopting immediate and reversible.