In records of tracheal pressure or flow taken from anesthetized cats appear large pressure-negative, or air inflow excursions, in response to single pulses applied to the central end of the vagus nerve. These responses have been attributed to phasic bronchodilation and not only due to the brief contractions of inspiratory muscles that occur as part of the total responses. Phasic bronchodilation responses appear mainly during inspiration but they may also occur in the expiratory phase through facilitation, during post-hyperventilatory apnea or that induced by constant current (d.c.) vagal stimulation. They are significantly reduced after lung sympathectomy. These bronchomotor responses showed long term depression (LTD) after spontaneous or reflexly evoked hiccups, and long term potentiation (LTP) after repetitive, or d.c. vagal stimulation. They are also potentiated on the experimental conditions that include exaggerated sympathetic activity. These and some other characteristics described indicate that bronchomotor responses are legitimate and can be used to follow the changes in excitability of the central generator of breathing.