We tested the hypothesis that lung inflation in response to a bronchospasm could be beneficial by maintaining the lung collateral channels open. Six mild asthmatics were studied on 2 separate days. The first day we determined the methacholine dose response and measured collateral resistance (Rcoll) before and during the metacholine-induced bronchospasm and the effects of decreasing lung volumes on Rcoll. The lung volume changes were induced by applying progressively increasing positive extrathoracic pressures (PEP). The second day we measured the changes in end-expiratory lung volume (EELV) resulting from the inhaled methacholine and from the applied positive extrathoracic pressure. With the inhalation of methacholine, the FEV1 decrease ranged from 26-43% of the baseline values while Rcoll increased significantly in only three of the six subjects. EELV remained unchanged in one subject and increased by 1408, 990, 260, and 44 ml in four others. It was not measured in one subject. Decreasing EELV by PEP increased Rcoll in all subjects. By extrapolation of the lung volume-Rcoll relationship, we calculated that Rcoll would have increased by 18,227%, 6843%, 994%, 140%, and 128% if EELV had not increased in the five subjects in whom delta EELV was measured. We conclude that an increase in EELV in response to an induced bronchospasm helps maintain open and functional collateral pathways despite the bronchoconstriction.