Previous studies have shown that angiotensin II (Ang II) can activate cardiovascular neurons within the medulla oblongata via an action on specific receptors. The purpose of this study was to determine the distribution of neurons within the medulla activated by infusion of Ang II into the fourth ventricle of conscious rabbits, using the expression of Fos, the protein product of the immediate early gene c-fos as a marker of neuronal activation. Experiments were done in both intact and barodenervated animals. In comparison with a control group infused with Ringer's solution alone, in both intact and barodenervated animals, fourth ventricular infusion of Ang II (4 to 8 pmol/min) induced a significant increase in the number of Fos-positive neurons in the nucleus of the solitary tract and in the rostral, intermediate, and caudal parts of the ventrolateral medulla. Double-labeling for Fos and tyrosine hydroxylase immunoreactivity showed that 50% to 75% of Fos-positive cells in the rostral, intermediate, and caudal ventrolateral medulla and 30% to 40% of Fos-positive cells in the nucleus of the solitary tract were also positive for tyrosine hydroxylase in both intact and barodenervated animals. The distribution of Fos-positive neurons corresponded very closely to the location of Ang II receptor binding sites as previously determined in the rabbit. The results indicate that medullary neurons activated by Ang II are located in discrete regions within the nucleus of the solitary tract and ventrolateral medulla and include, in all of these regions, both catecholamine and noncatecholamine neurons.