Previous studies have shown developmental differences in smooth muscle tone of the airways. Differences in airway mechanics may be based upon cellular differences between animals of different ages. We developed a method [Driska et al. J Appl Physiol 86(1): 427–35, 1999] for isolating ovine tracheal smooth muscle cells and for measuring shortening velocity. This technique was used to study the change in contractile response of the airway smooth muscle (ASM) cell during development. Here we report differences between preterm (PT) (110–124 or 125–140 days post-conception), newborn (NB) (3–7 days postnatal) and adult (9–36 months) cells. These cells were compared with respect to morphometry, shortening velocity, and percent shortening. The neonatal cells were shorter and narrower than the adult cells. Maximum shortening velocity was faster for adult (45.1 μm/sec) than for neonatal cells (range 11.1–25.1 μm/sec). When velocity was normalized to the cell length, there was no difference between the adult and PT cells, but there was a significant difference between the NB (0.30 sec−1) and adult (0.54 sec−1) cells. The percent shortening did not show any significant difference with age. Within the neonatal groups, there were no significant differences in morphometry, shortening or velocity. To facilitate comparison between ASM tissues of different sizes with different sized cells, we also expressed percent shortening and velocities relative to a hypothetical 1 mm segment of tissue. Represented this way, the amount of shortening for all age groups was the same, but the predicted maximum velocity of the hypothetical PT tissue (125–140 days) was significantly greater than for NB.