The present study examined the location of neurons in the lateral parabrachial nucleus (PBL), Kölliker-Fuse nucleus (KF), nucleus of the solitary tract (NTS), spinal trigeminal nucleus (Sp5C) and upper cervical cord possibly involved in the transmission of autonomic responses (apnea, bradycardia and rise in arterial blood pressure) elicited by nasotrigeminal stimulation in the rat. To identify these neurons we employed immunocytochemical detection of the transcription factor Fos. To induce the expression of Fos protein, two kinds of stimuli and experimental controls were performed in chloralose/urethane-anesthetized animals: (i) electrical stimulation of the trigeminal ethmoidal nerve (EN5) and, as sham controls, dissection of the EN5 without electrical stimulation, (ii) stimulation of the nasal mucosa with water and, as control experiments, no stimulation. Both forms of stimulation lead to a consistent pattern of Fos-positive neurons in the PBL and KF. Differences could be observed rostrally in the PBL and KF, where significantly higher numbers of Fos-positive neurons were present after EN5 versus water stimulation. The EN5-stimulated group had a significantly higher number of Fos-immunoreactive neurons in the KF than the sham controls, especially in the midlevel region. In the PBL significant differences in the numbers of activated cells could be observed between EN5-stimulated versus sham controls. In the water-stimulated rats compared with the anesthesia controls, a significantly higher number of Fos-immunoreactive neurons was always observed at all rostrocaudal levels in the KF and in the midlevel PBL. Electrical EN5 stimulation induced Fos expression in the Sp5C at the level of the area postrema and caudally in the upper cervical cord. In contrast, after water stimulation Fos-positive neurons were exclusively found in the Sp5C. In addition, all forms of stimuli and controls induced strong expression of Fos in the medial and commissural NTS. Linear correlations were found between the numbers of Fos-immunoreactive neurons in the Sp5C versus the KF and the NTS versus the PBL. The activated neurons may belong to two functionally discrete pathways: the nasotrigeminal reflex circuit, which is activated by nasal sensory afferents running through the EN5 via the Sp5C to the KF, and a pathway activated most likely by baro- and chemoreceptor afferents running through the NTS to the PBL. Our results indicate that the PB/KF plays a pivotal role in the mediation and maintenance of the autonomic responses induced by the nasotrigeminal reflex.