Respiratory activity was recorded on hypoglossal nerve or ventral cervical roots during in vitro experiments performed in the superfused brainstem-cervical cord preparation of newborn rats. Section and coagulation experiments revealed that the medullary respiratory generator was tonically inhibited by a structure located in the caudal ventrolateral pons. Electrical and pharmacological stimulations located this structure more precisely between the superior olivary nuclei and the sensory nucleus of the Vth nerve, i.e. in an area containing the A5 noradrenergic nucleus. Norepinephrine and alpha 2-antagonists (yohimbine, idazoxan) added to the bathing medium modified the respiratory frequency. Norepinephrine decreased respiratory frequency whereas norepinephrine antagonists increased respiratory rate. The electrical stimulation of the caudal ventrolateral pons which inhibited the respiratory rhythm under normal bathing medium became ineffective after alpha 2-antagonist. The results herein suggest that a noradrenergic inhibitory drive, originating from the A5 area or surrounding structures modulates the activity of the medullary respiratory generator. This hypothesis is discussed in relation to A5 involvement in cardiovascular regulation.