Messenger RNA encoding the immediate early genes (IEGs) c-fos and NGFI-A was localized by in situ hybridization of specific 35S-labelled oligonucleotides to detect activated neurones in the medulla oblongata following unilateral electrical stimulation of the vagus (nX) and aortic depressor nerve (ADN), and following mechanical stimulation of the left carotid sinus (CS). In electrically stimulated rats, c-fos and NGFI-A mRNA was strongly expressed in the nucleus tractus solitarius (NTS) (predominantly ipsilaterally), area postrema (AP) and in a dorsal subregion of the paratrigeminal nucleus (PTN). Lower levels of c-fos and NGFI-A mRNA were seen in the ipsilateral NTS and PTN following mechanical stimulation of the left CS. In general these data correlate with the topography of innervation by the different nerve afferents, although the expression in the PTN (and in some cases the AP) would not be predicted on the basis of neuronal innervation patterns reported for the rat. Expression of these IEGs also occurred in the rostral and caudal ventrolateral medulla and inferior olive of both stimulated and sham-operated rats; presumably due to effects of the anaesthesia and surgical procedures. In conclusion the localization of the expression of c-fos and NGFI-A mRNAs represents a useful neuroanatomical technique for detecting the cell bodies of neurones that are activated by cardiovascular nerve afferents and should allow the further characterization of the neurochemical identity of these neurones.