The influence of vagal stretch receptor afferents on respiratory motor-output and respiratory changes in esophageal pressure (DeltaP(es)) was studied in anaesthetized cats. Tracheal occlusions and lung inflations were performed during hyperoxic normocapnia, during electrical stimulation of one carotid sinus nerve (CSN) or the intracranial medullary chemosensitivity (MCS), during hypercapnia or the combination of CSN and hypercapnia. Tracheal occlusions during inspiration led to increased and prolonged inspiratory muscle (IM) activity. Moderate hyperinflation in inspiration decreased and shortened inspiratory motor output. Changes in esophageal pressure and in amplitude and discharge duration of IM are largely proportional (0.84>r<0.98) to lung volume above normal endexpiratory volume (FRC). The effects are described as the Hering-Breuer inspiration inhibitory reflex (HB-IIR). Tracheal occlusion or hyperinflation in end-inspiratory position not only prolonged expiration but also activated expiratory muscles (EM). The effects linearly (0.86>r<0.98) increased with elevation of lung volume. We refer to these effects as the Hering-Breuer expiration facilitatory reflex (HB-EFR). Severe hyperinflation or rapid inflation of the lungs during inspiration, however, led to an inspiratory facilitation with increased IM activity. During concomitant chemoreflex activation, CSN or MCS stimulation, respiratory hypercapnia, or the combination of both, the extent of the above described responses of IM and EM activity were significantly (0.05>p<0.0002) enlarged. The changes in the discharge period of IM and EM following lung inflation were smaller in the presence of the increased chemical respiratory drive (0.01>p<0.005). The relative changes in EM responses to lung inflations during increased respiratory drive were greater than those of IM. Bilateral vagotomy abolished the respiratory responses to tracheal occlusion and hyperinflation of the lungs. The results of the present investigation show that aside from the well-known inhibition of inspiration, vagal slowly adapting lung stretch receptors facilitate expiration. The sensitivity of the lung reflexes is enhanced with increasing respiratory drive. The HB-inspiration inhibitory reflex limits the depth of lung inflation, whereas the HB-expiration facilitatory reflex promotes an effective lung deflation. Both reflex mechanisms, the inspiratory and expiratory one, are present in eupnoeic breathing, but play an important role during increased chemoreflex drive and obstruction of expiration, e.g., with increased external airway resistance.