Changes in airway resistance are reported to account for only a portion of changes in total lung resistance. The fraction of total lung resistance caused by airway resistance is difficult to quantify in vivo. High-resolution computed tomography (HRCT) has potential application for directly measuring changes in airway size in vivo. In the present investigation, we studied five anesthetized mongrel dogs using HRCT to locate and measure changes in airway area after aerosol histamine challenge in the absence and presence of deep inspiration. We also related changes in total lung resistance to changes in airway area. We found that in all dogs after histamine aerosol challenge, airway area decreased (range, 23 +/- 7 to 67 +/- 5%, mean +/- SEM), and total lung resistance increased (range, 191 to 378%). After deep inspiration (equal to three times tidal volume), four of the five dogs showed further significant decreases in airway area (range, 13 +/- 6 to 71 +/- 8%), whereas all five dogs showed decreases in RL (range, 3 to 35%). The fact that preconstricted airways constricted further after deep inspiration while the measured RL decreased suggests that RL may not always be a reliable indicator of changes in the size of conducting airways larger than 1 mm.