The Retrotrapezoid/parafacial Respiratory Group (RTN/pFRG) contains late-expiratory (late-E) and CO2-sensitive neurones involved with active expiration and central chemoreception, respectively. Here we evaluated whether or not RTN/pFRG late-E and CO2-sensitive neurones present distinct electrophysiological properties and CO2 sensitivity. Using an in situ working heart brainstem preparation of rats (7 weeks old), we performed recordings of abdominal (AbN) and phrenic (PN) nerves activities simultaneously with whole cell path-clamp recordings of late-E or CO2-sensitive neurons in the RTN/pFRG. The data shows that late-E and CO2-sensitive neurones constitute heterogeneous neuronal populations with respect to several electrophysiological properties. During normoxic/normocapnic condition the late-E neurones were silent, while CO2-sensitive displayed continuous frequency discharge (0.83 ± 0.02 Hz; n=8) during the respiratory cycle. The emergence of late-E activity, in late-E neurons, and the coincidently late-E burst in AbN motor activity, both preceding PN bursts, occurred only during hypercapnia (10% CO2; 9.1 ± 0.08 Hz; n=7) or hypoxia (5% O2; 8.5 ± 0.09 Hz; n=7). Input resistance (354 ± 21 vs 235 ± 17 MΩ; p<0.05), resting membrane potential (-67 ± 3 vs -54 ± 2mV; p<0.05) and rheobase (35 ± 3 vs 58 ± 7 pA; p<0.05) were different, suggesting that the expression or properties of some ion channels differ between these neuronal types. Common features of RTN/pFRG late-E and CO2-sensitive neurones included the expression of transient outward potassium current (TOC; at 60 mV: 365 ± 65 vs 380 ± 57 pA/pF), hyperpolarization-activated inward current (at -120 mV: 35 ± 3 vs 47 ± 7 pA/pF) and absence of persistent sodium current. Nevertheless, after synaptic blocked, hypercapnia increased the firing frequency (+1.83 ± 0.05 Hz) and decreased the potassium-mediated leak current (- 3.1 ± 1 nS) of CO2-sensitive neurones, but did not change the late-E neurones activity. This body of electrophysiological evidence indicates that RTN/pFRG late-E and CO2-sensitive neurones from juvenile/adult rats constitute heterogeneous neuronal populations regarding intrinsic electrophysiological properties and CO2 sensitivity, and provide a cellular basis for a better understanding of their involvement in the pathophysiological conditions, such as obstructive sleep apnoea, hypertension and heart failure.