1 In "encephale isole" cats with a spinal section in C7, we have recorded the activity of 108 medullary inspiratory or expiratory neurones during normopnea or polypnea induced by local warming of the anterior hypothalamic region. The neurones were grouped according to their axon destination, their discharge pattern, and the correlation level between their unit activity and the phrenic neurogram. 2 In polypnea, 71% of inspiratory neurones remain active. Their firing rates, which increase during inspiratory discharges in normopnea, decrease during inspiratory discharges in polypnea. However, this inversion of discharge pattern is less marked than with phrenic motoneurones. 3 The reduction of activity in polypnea preferentially strikes certain types of neurones. While bulbo-spinal inspiratory neurones all remain active, the propriobulbar inspiratory neurones can be grouped into those which remain active and whose activity is well correlated with the phrenic neurogram, and those which become silent and showing a poor correlation. As a general rule, activity in polypnea and correlation (in normopnea) go together. 4 In polypnea, 56% of expiratory neurones cease discharging. This activity decrease applies to bulbo-spinal expiratory neurones (5 out of 14) and to propriobulbar expiratory neurones (20 out of 31). Among the expiratory neurones, only those with an elevated discharge firing rate and significant correlation coefficient values remain active in polypnea. At this time they generally exhibit a decreasing firing rate and emit only a few spikes in each burst. 5 The results suggest that during polypnea the genesis of the respiratory rhythm is dependent upon a different mechanism than in normopnea.