The effects of a single systemic injection of reserpine on c-fos proto-oncogene expression in catecholaminergic neurons of the rat brainstem were studied by immunohistochemistry for Fos proteins (Fos). In control rats, a few Fos immunoreactive neuronal nuclei were observed in the tectum and mesencephalic central gray. Within hours after drug injection, a substantial number of brainstem neurons stained intensely for Fos. The staining was maximal at 6 h and returned to control levels within 24 h. Double-immunohistochemical staining with antibodies to tyrosine hydroxylase revealed that in all noradrenergic (NA) neuron subgroups except the A2 group, the majority of NA neurons stained for Fos. Most adrenergic neurons were also labeled. In contrast, aside from some cells in the ventral tegmental area, reserpine did not induce Fos immunoreactivity in dopaminergic neurons. Numerous non-catecholaminergic neurons were intensely stained with Fos in the substantia nigra pars reticulata, ventral tegmental area, mesencephalic central gray, pontine nuclei and tectum. A small number of Fos immunoreactive neurons was also observed in raphe nuclei. Injection of saline (i.p.) resulted in a moderate increase in Fos immunoreactivity in the locus ceruleus, in A1/C1 neurons and in the mesencephalic central gray. The results demonstrate that acute reserpine treatment induces Fos expression in distinct populations of brainstem neurons, comprising both catecholaminergic and non-catecholaminergic neurons. Thus, induction of Fos by reserpine does not coincide with the site of action of this drug. The distribution of Fos immunoreactive NA neurons after reserpine treatment is comparable to that reported after application of stressful stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)