Aside for the potential for tonic contraction, the airway smooth muscle exhibits intermittent phasic rhythmic activity that may contribute to lung growth during fetal life. Therefore, we examined 4th generation rat 18 -22 d gestation fetal, 4 -6 d of age newborn and adult bronchial ring from Sprague Dawley rats to compare differences in smooth muscle function. We hypothesized that phasic contractions were greatest before birth. Bronchial muscle spontaneous rhythmic contractions were greatest in the fetus and absent in the adult. In response to KCl stimulation, the fetal bronchial smooth muscle only developed tonic force that was 3.5 Ϯ 0.6 and lower than measured in the newborn 9.0 Ϯ 0.3 and adult 13.7 Ϯ 1.4mN/mm 2 . The thromboxane A 2 analogue U46619 induced tonic and phasic muscle contractions and the amplitude and frequency of the phasic contractions were greater in the fetus as compared with the adult and increased with gestational age. The U46619-induced rhythmic contractions were abrogated by ryanodine, thapsigargin and reduction of extracellular Na ϩ , suggesting intracellular Ca 2ϩ dependence and involvement of the Na ϩ /Ca 2ϩ exchanger. The inward rectifier K ϩ blocker BaCl 2 induced phasic contractions in unstimulated fetal, but not adult bronchial muscle of the same amplitude and frequency as for the spontaneous and U46619-induced ones. We conclude that the airway smooth muscle phasic activity is greatest in the fetus and tends to disappear post-natally with age suggesting an in utero role during lung development.