Previously we showed the shortening velocity of guinea pig tracheal strips was the greatest in juvenile (3-week old) compared to infant (1-week old) and adult animals (3-month old). The greatest shortening velocity was associated with the least resistance to shortening calculated from force-velocity curves among the 3 age groups. It remained to be verified if the stiffness of tracheal tissue, a measure of tissue response to geometrical deformations, is different among the 3 age groups. We hypothesized that the stiffness of intact tracheal strips is the lowest in the juvenile group and that this can explain the ontogeny of airway smooth muscle resistance to shortening and shortening velocity. Static stiffness measured through stepwise deformations showed no age-related differences. Evaluation of tissue response to oscillatory deformations showed that the dynamic stiffness of unstimulated tracheal strips was 8.35±0.88, 4.15±1.09, and 8.21±1.57 kPa, and the phase angle, a measure of tissue viscosity, was 10.3±2.93, 2.46±0.67, and 7.87±1.77 degrees in infant, juvenile, and adult respectively. Unstimulated juvenile strips were significantly lower in dynamic stiffness (p<0.05) and phase angle (p<0.05) in comparison to unstimulated infant or adult strips. This maturational profile was independent of muscle strip pre-set length or oscillation mode/amplitude, but was abolished at the peak of contraction to either carbachol or electric field stimulation. These results suggest that the non-contractile components of tracheal strips are less stiff and contain fewer viscous/frictional elements in juvenile than in other age groups. This may provide a functional basis for reduced resistance to length changes in juvenile airway smooth muscle.