Systemic administration of the cytokine interleukin-1 (IL-1) results in increased secretion of ACTH and corticosterone in rats. The available evidence suggests that the acute effects of IL-1 are exerted ultimately at the level of the hypothalamus to increase corticotropin-releasing factor (CRF) secretion into the hypophyseal portal circulation, and hence the central drive on the pituitary-adrenal system. However, the route(s) and mechanism(s) by which circulating IL-1 gains access to central mechanisms governing pituitary-adrenal output remain poorly understood. In this study, we show that intravenous injection of IL-1 beta provokes time- and dose-dependent increases in the expression of the immediate-early gene c-fos, in identified CRF and oxytocin- producing cells of the paraventricular nucleus of the hypothalamus (PVH). Several cell groups known to be involved in central visceromotor regulation also displayed comparable time- and dose-related activation to systemic IL-1, including the bed nucleus of the stria terminalis, the central nucleus of the amygdala, the lateral parabrachial nucleus, and cell groups of the dorsomedial and ventrolateral medulla. Activation of circumventricular organs, which have been hypothesized to serve as central monitors of circulating IL-1, required doses roughly an order of magnitude above those required to activate CRF neurons in the PVH. Combined immunohistochemical and retrograde tracing experiments revealed many IL-1-responsive cells in the nucleus of the solitary tract and the ventrolateral medulla to be catecholaminergic and to project to the region of the PVH. Discrete and unilateral interruption of ascending catecholaminergic projections from the medulla attenuated IL-1-stimulated increases in Fos immunoreactivity and CRF mRNA in the PVH on the ipsilateral side. Disruption of descending projections from circumventricular structures associated with the lamina terminalis did not affect IL-1-mediated Fos induction in the PVH. We conclude that medullary catecholaminergic projections to the PVH play either a mediating or a permissive role in the IL-1- induced activation of the central limb of the hypothalamo-pituitary- adrenal axis.