The role played by both lung stretch and lung deflation components of the pulmonary vagus has been studied, in order to throw further light on the changes in lung mechanics which underlie the lung deflation receptor activity and hence the activation of respiration during an asthma attack. The discharge pattern of the lung stretch receptors varies from unit to unit, but also in one and the same unit during an asthma attack. The heterogeneous pattern of the stretch receptor units seen during the latter indicates uneven ventilation: the increased stretch receptor activity during both phases of respiration points to overinflation of the lung units concerned during inspiration, and to insufficient relaxation during expiration; the decreased stretch receptor activity in inspiration, combined with deflation receptor discharges in expiration, would indicate that the lung units concerned are poorly inflated during inspiration and subjected to forced deflation during expiration. The reflex mechanisms underlying the activation of respiration during an asthma attack are discussed.