•  The nucleus incertus (NI) is a stress and arousal responsive, hindbrain region involved in ascending control of septohippocampal theta rhythm. •  NI neurons express high levels of the neuropeptide relaxin‐3 and corticotrophin‐releasing factor (CRF) receptor‐1 (CRF‐R1). •  We report the first in‐depth characterization of NI neurons, using in vivo and in vitro electrophysiological techniques, which reveal a population of relaxin‐3‐containing NI neurons activated by CRF via postsynaptic CRF‐R1 and a non‐relaxin‐3 neuron population inhibited or unaffected by CRF. •  Relaxin‐3 NI neurons exhibit strong phase‐locked firing with the ascending phase of hippocampal theta oscillations. •  These findings suggest the NI is a heterogeneous neuronal population and key site of CRF action with the capacity to modulate cognition in response to stress. Abstract  The nucleus incertus (NI) of the rat hindbrain is a putative node in the ascending control of the septohippocampal system and hippocampal theta rhythm and is stress and arousal responsive. NI contains GABA neurons that express multiple neuropeptides, including relaxin‐3 (RLN3) and neuropeptide receptors, including corticotrophin‐releasing factor receptor‐1 (CRF‐R1), but the precise anatomical and physiological characteristics of NI neurons are unclear. Therefore, we examined the firing properties of NI neurons and their responses to CRF, the correlation of these responses with occurrence of relaxin‐3, and NI neuron morphology in the rat. Most NI neurons excited by intracerebroventricular CRF infusion were RLN3‐positive (9 of 10), whereas all inhibited cells were RLN3‐negative (8 of 8). The spontaneous firing of RLN3 (n= 6) but not non‐RLN3 neurons (n= 6) was strongly modulated and phase‐locked with the initial ascending phase of hippocampal theta oscillations. In brain slices, the majority of recorded NI neurons (15 of 19) displayed excitatory responses to CRF, which uniformly increased action potential frequency and membrane potential depolarization in the presence of tetrodotoxin, indicating a direct, postsynaptic action of CRF on NI neurons. This excitation was associated with reduction in the slow component of afterhyperpolarization and a strong depolarization. Quantitative analysis in naïve rats of validated CRF‐R1, RLN3 and neuronal nuclear antigen (NeuN) immunoreactivity revealed 52% of NI neurons as CRF‐R1 positive, of which 53% were RLN3 positive, while 48% of NI neurons lacked CRF‐R1 and RLN3. All RLN3 neurons expressed CRF‐R1. CRF neurons that projected to the NI were identified in lateral preoptic hypothalamus, but not in paraventricular hypothalamus, bed nucleus of stria terminalis or central amygdala. Our findings suggest NI is an important site for CRF modulation of hippocampal theta rhythm via effects on GABA/RLN3 transmission.