Systemic hypoxia stimulates the release of vasopressin (VP) and adrenocorticotropin hormone (ACTH). To examine the involvement of catecholamine cell groups of the ventrolateral medulla (VLM) in the neuroendocrine responses, we have used the c-fos activity mapping technique to compare the effects of hypoxia on VLM catecholamine cells to those on neurosecretory VP and putative corticotropin releasing factor (CRF) containing cells. A limited degree of catecholamine cell activation was evident at predominantly mid-VLM levels at 12% oxygen in the inspired air. Further reduction in inpsirate oxygen levels enhanced recruitment of caudally located VLM catecholamine cells considered to form part of the A1 noradrenergic cell group. Threshold for activation of VP and putative CRF cells occurred at the 10% oxygen level. Unexpectedly, this stimulus also activated neurosecretory oxytocin (OT) cells. With increasing hypoxic severity the number of activated supraoptic VP and OT cells was not significantly different to that observed at the 10% level. However, paraventricular neuroendocrine responses continued to increase with putative CRF containing cells of the medial parvocellular zone having nearly double the level of activity (as measured by the number of cells within this region displaying Fos-like immunoreactivity; FLI) at 6% compared to that apparent to the 10% level of hypoxia. Paraventricular VP cells displaying FLI were also increased at the most severe levels of hypoxia but this effect was much less marked than the medial parvocellular response. Consistent with a role for VLM catecholamine cells in generation of neuroendocrine cell responses to hypoxia, unilateral VLM lesions, restricted to the caudal two thirds of the catecholamine cell column, resulted in significant reductions in the responses of all three cell types. These results, in addition to establishing a role for VLM catecholamine cells in neuroendocrine cell responses to systemic hypoxia, have important general implications for catecholamine cell group involvement in neuroendocrine regulation.