Frequency dependence of thoracic gas volume (Vtg) and phase angles between volume and mouth pressure signals were evaluated in 10 normal subjects and in 31 patients with bronchitis, who panted sequentially at 0.8 Hz and 2.5 Hz during the same occlusion. A slight negative frequency dependence of Vtg (-1.2 +/- 0.9%/Hz, -m +/- SD), accompanied by a very small phase lag of volume with respect to pressure variations (-1.5 +/- 0.9 degrees at 2.5 Hz) was found in normal subjects. In contrast, patients with bronchitis exhibited a mild positive frequency dependence of Vtg (+ 0.5 +/- 1.7%/Hz and + 0.8 +/- 2.3%/Hz in subjects with normal and increased airway resistance, respectively), whereas the volume signal slightly led the pressure signal at 2.5 Hz. Both the frequency dependence and the phase difference increased when the subjects with the largest airway resistance did not support their cheeks during the panting maneuvers. The data confirm previous indirect evidence that Vtg is, in most instances, accurately measured by Boyle's law in mild and moderate chronic airway obstruction.