In his 1954 Harvey lecture on the functions of the lung, Comroe divided pulmonary reflexes into regulatory and defensive or protective (18). Char acteristically, regulatory reflexes are subject to a continuous input from low-threshold afferent endings whose impulse frequency waxes and wanes as their effective stimulus (eg volume or pressure) varies around a control or resting setpoint. Reflexes of this type exert a continuous control of ventilatory events (11) and promote the smooth interaction of the ventilatory and cardiovascular systems (13), their predominant function being to main tain the status quo. Defensive reflexes are a broader constellation of reflex responses evoked by stimuli that threaten lung function. These reflexes are the subject of the present review. Both regulatory and defensive reflexes are mediated predominantly by afferent fibers traveling to the medullary centers in the vagus nerves. The sensory innervation of the lower airways includes a component whose cell bodies are in the upper thoracic dorsal root ganglia and whose fibers travel to the spinal cord with branches of the sympathetic nerves. However, these so-called sympathetic afferents appear to play a relatively minor part in pulmonary defensive reflexes, and even powerful chemical stimuli applied to their endings cause little more than a brief disturbance of breathing (14, 19). Pulmonary defensive reflexes (or effects indistinguishable from them) are not only triggered by extrinsic agents such as irritant gases that pose a threat to the airways, but can be