Immature airways ape highly compliant compared to the adult, suggesting that trachea and bronchi from immature animals may be easily compressed. Although tracheal compression has been extensively studied, the effect of transmural pressure an occlusion of immature bronchi has been neglected.The transmural pressure at which the lumen closed was determined from the transmittance of pressure along the lumen of isolated bronchi from late-term foetal, 1 and 4 week old pigs. Bronchi from eight cases of sudden infant death syndrome (SIDS) were also studied. In several experiments, smooth muscle tone was produced either by electrical field stimulation of carbachol challenge, and the relationship between active muscle tone and resting transmural pressure was studied.Bronchi from foetal, 1 and 4 week old pigs were occluded by intraluminal pressures of -4, -5 and -24 cmH(2)O. SIDS bronchi closed at -11 cmH(2)O. Histological and endoscopic investigations showed that closure of the bronchi occurred along a plane and was not uniform along the bronchus. Carbachol precontraction increased the transmural pressure required to close bronchi by approximately 5 cmH(2)O. The relationship between muscle tone and resting pressure was the same in all age groups, except when transmural pressure was at or below closing pressure.Bronchi from immature animals and human infants are vulnerable to collapse by small changes in transmural pressure. Bronchial closure is partly dependent on smooth muscle tone, particularly in younger animals.