To reveal central nervous system (CNS) structures involved in the baroreceptor reflex we studied the distribution of Fos protein-like immunoreactivity in the rat brain after one hour of electrical stimulation of the aortic depressor nerve (ADN). In 13 male Wistar rats under urethane the ADN was cut on both sides and the central ends were placed on stimulating electrodes. Intermittent (11 s on, 6 s off) electrical stimulation at parameters set to elicit a drop in mean arterial pressure of 15-30 mmHg was applied to one, both or neither ADNs for 1 h. CNS sections were incubated for 48 h in anti-Fos antibody and prepared for visualization of the reaction product using the ABC immunoperoxidase technique. Label was found in several discrete brain nuclei primarily on the side ipsilateral to the side of stimulation. In the medulla labelled nuclei were found in the nucleus tractus solitarius, area postrema, rostral and caudal ventrolateral medulla, nucleus ambiguus and medullary reticular formation. In the pons labelled neurons were found in the lateral and ventrolateral parabrachial nucleus, locus coeruleus, pontine reticular field and A5 region. In the forebrain labelled nuclei were observed in the peri- and paraventricular hypothalamus, supraoptic nucleus, subfornical organ, preoptic area, central nucleus of the amygdala, median preoptic area, horizontal limb of the diagonal band, bed nucleus of the stria terminalis and islands of Calleja. In control animals moderate amounts of label were present in the supraoptic nucleus and periventricular hypothalamus bilaterally. These results define central pathways involved in mediating the baroreceptor reflex.