Respiration related units (RRU) were recorded during a stratigraphic exploration of medulla and pons from the cervical junction to the caudal part of the pneumotaxic system in the semi-chronic locally anesthetized 'isolated respiratory centre' of the cat. Metal 'low impedance, capacitance compensated' microelectrodes recorded multi-unit signals from which unitary discharges were discriminated and processed by computer; it is suggested that using these techniques, the sample was a good representation of the total unit population. The phase relation to phrenic discharge was determined on cycle triggered time histograms. Of 23,000 units, 28% had a definite respiratory modulation. Examined individually, each RRU showed a stable discharge pattern corresponding to one of various respiratory types, the majority of which have been described previously. Both temporal and spatial distributions of RRU discharges were analyzed. Temporal distribution of peak firing frequencies (PFF) of 5,000 RRU sampled anatomically at random showed two main populations whose modes were observed during inspiration (I) or expiration (E). Troughs were observed in the histogram at the transition from I to E and E to I, thus indicating low probability for finding phase spanning RRU in the medulla and pons up to the pneumotaxic level. These statistical results turned out to be identical to those obtained with an a priori classification method comparable to that used in most of the previous works. In addition, the PFF distribution suggested that the E population could be further divided into 3 sub-populations whose modes fall in early, mid, and late expiration respectively. Comparison of RRU temporal distribution in two regions, one rostral, another caudal to a frontal Horsley-Clarke plane situated 3 mm in front of the obex, showed that, in the caudal region, 70% of the RRU were I units, while, in the rostral medulla and pons, equal proportions of I and E neurons were found. Temporal distribution of RRU peak frequencies was studied separately in anatomical structures where the probability of finding RRU was high. No clear correspondence between RRU types and anatomy could be found, but marked differences between structures were observed, thus suggesting nevertheless a different spatial distribution for I and E populations.