To identify within breath variations in the mechanical properties of the isolated upper airway, we examined changes in impedance spectra over the course of the respiratory cycle. Changes were evaluated with a modified forced oscillation technique applied to the isolated, sealed upper airways of nine anesthetized mongrel dogs. Upper airway impedance spectra were studied during sequential 350 msec epochs. We found spectral changes which were reproducible within the respiratory cycle. Impedance spectra revealed that during mid-inspiration at the point of peak upper airway muscle activity, the low frequency real part decreased and the imaginary part was less negative and less steep. During late inspiration and early expiration the impedance values returned to their end-expiratory values. The only significant change in parameter estimates from a three-parameter model indicated an increase in compliance. Since these changes correlated not only with tidal flow through the lower trachea and lung but also with upper airway muscle activation, we reasoned that changes in impedance could have resulted from an increase in upper airway size. Therefore, we used a sealed speaker system and, while the animal was apnoeic, evaluated impedance at two different airway pressures and the resultant volumes. The changes in impedance spectra with a volume increase were similar to those seen during spontaneous breathing efforts. We conclude that the mechanical properties of the upper airway change during the respiratory cycle and that these changes correlate with the respiratory activation of upper airway muscles. We suspect that these changes in input impedance could reflect a change in the size of the airway rather than a true increase in elasticity.