10 The activity of 107 medullary inspiratory neurones has been recorded extracellularly in anesthetized cats (urethane-chloralose). According to their localization in the medulla and to their axonal pathways (tested by antidromic activation), these neurones were classified as: bulbo-spinal neurones (NBSI) which send their axons to the spinal cord; they are located in the dorsal or the ventral respiratory nucleus; propriobulbar neurons (NPBI) whose axons are probably entirely located within the medulla; they are found only in the ventral nucleus. 20 Summation of gross discharges from phrenic (C5 ventral root) or inspiratory intercostal (from T4 to T10) nerves was triggered by spikes from medullary respiratory neurones. If the studied neurone controls the activity of the recorded motor fibres, after sommation of a great number of sweeps (3,000 to 5,000), the summed nerve activity displays a short delay excitatory wave, indicating that the activity of the motoneurones is increased after the onset of the medullary potential. On the other hand, there is a flat summed nerve activity if the nerve discharge is not directly dependent on the medullary recorded activity. 30 The dorsal nucleus NBSIs send their axon to the contralateral phrenic motoneurones (as it is revealed by the excitatory wave noted on the summed phrenic nerve activity, fig. 1 and 4) and not to the intercostal motoneurones (fig. 4). 40 The ventral nucleus NBSIs control contralateral intercostal motoneurones (fig. 5). Some of them send their axons only to these motoneurones, but the majority also act on phrenic motoneurones (fig. 2 and 5). 50 Connections between NBSIs and phrenic motoneurones are monosynaptic and probably also those between NBSIs and intercostal motoneurones. 60 No excitatory wave has been observed on phrenic or intercostal summed activity when summation was triggered by NPBI spikes (fig. 3). This confirms the accuracy of the "antidromic stimulation test" used to classify medullary respiratory neurons.