The retrotrapezoid nucleus (RTN), a part of the rostral ventrolateral medulla, is involved in the control of breathing. A recent immunohistological study suggested a possible involvement of the RTN in hypoxic chemoreflex loop. The present electrophysiological study performed in the cat demonstrates that 23 out of 24 RTN neurons were stimulated during the biphasic respiratory response to hypoxia, which consists of a reinforcement followed by a depression of respiratory activity. This confirms the previous immunohistological study. While 15 RTN neurons might integrate either phase I (n = 7) or phase II (n = 8) O2-chemosensitive inputs, the remaining eight RTN neurons stimulated by hypoxia are susceptible to integrate both phase I and phase II O2-chemosensitive inputs. In conclusion, our results suggest that the different subsets of RTN neurons may influence respiratory output by conveying signals originating from peripheral and/or central chemoreceptors stimulated during hypoxia.