.-Bronchoconstrictor responses are quantitatively different when they are evoked under static conditions and during or after periods of deep inspiration. In vivo, deep inspirations produce bronchodilation and protect the lung from subsequent bronchoconstriction (termed bronchoprotection). These effects may be due in part to dynamic stretch on airways produced by cyclical expansion of airway diameter. However, airways also lengthen cyclically during breathing. The effects of cyclical airway elongation on evoked bronchoconstriction have not been examined. This study recorded evoked contractions of pig bronchial segments 1) at different airway lengths, 2) after a period of cyclical lengthening in relaxed airways, and 3) during cyclical lengthening in pretoned airways. Airway segments were mounted in organ baths and bathed in Krebs solution luminally and on the adventitia. Airways were cyclically lengthened by 5-30% of their deflated length at 0.5-2 Hz for 5 min. Contractions were evoked by electrical field stimulation or carbachol and were recorded under isovolumic conditions. Under static conditions, there was a blunt relationship between length and response to electrical field stimulation. After a period of airway length cycling, electrical field stimulation-induced contractions were increased. In airways pretoned with carbachol, cyclical lengthening produced a transient bronchodilation and a sustained increase in contraction. Contractile responses were not blocked by indomethacin. The results show that isolated airways respond actively to dynamic changes in length. Our results indicate that cyclical lengthening of airways could contribute to lung function in vivo but does not appear to account for the phenomenon of bronchoprotection.bronchoconstriction; airway smooth muscle; deep inspiration LUNG VOLUME HAS A NUMBER of different effects on the capacity of airways to actively narrow. Under static or semistatic conditions, evoked bronchoconstriction is reduced at high lung volumes as a result of increased afterload on airway smooth muscle (ASM) (16,17). Conversely, at low lung volumes, bronchoconstriction is enhanced. Under conditions present during normal breathing or deep inspirations (DI), the respiratory system responds actively to the dynamic conditions present. For example, in normal subjects, periods of DI produced bronchodilation and, importantly, reduced bronchoconstriction that was evoked subsequently to the DI (5,14,18,22,24,28). The latter phenomenon has been termed bronchoprotection. Some authors (6) propose that bronchoprotection is one of the most powerful protectective mechanisms in the airway. Bronchoprotection has added significance because the phenomenon appears to be lost or even reversed in subjects with asthma (14, 24).The causes of bronchodilation and bronchoprotection are not fully understood but may involve the effects of cyclical force on ASM (6, 34, 36). With normal tidal breaths and DI, airways expand in diameter and also elongate (1, 12, 19). The effects of cyclical expansion on airway...