Abstract The effects of changes of trachealis active tension on the time-dependent pressure-volume relationships in saline filled segments of the in vivo canine intrathoracic trachea were studied. In the vagotomized segment at zero transmural pressure, rapid injection of a small quantity of fluid into the lumen resulted in stress relaxation occurring in a 4 second time period of 23.8 ± S.D. 6.1%. Similar experiments performed under conditions where trachealis muscle active tension was increased by constant electrical stimulation of the cervical vago-sympathetic trunk gave increases in four second stress relaxation to values as high as 80%. There was a linear relationship between 4 second stress relaxation and calculated active tension of trachealis muscle. Hysteresis of the segment also was a function of trachealis muscle active tension. Little or no change in 4 second stress relaxation was seen with increases in trachealis active tension in partially collapsed segments suggesting that the viscous properties of tracheal muscle have little effect on time-dependent pressure-volume relationships under conditions where the ends of tracheal cartilages are articulating. These data support the concept that time-dependent stress strain properties of airways are primarily a result of viscoelastic or viscoplastic properties of airway smooth muscle and are markedly influenced by changes in smooth muscle active tone.