The mRNA expression of the proinflammatory cytokine interleukin-1β (IL-1β) has been shown to be induced in neural elements during ischemia. It is not clear which cells generate the IL-lβ mRNA and eventually synthesize IL-1 protein and which cells respond to this signaling by producing IL-1 receptors during ischemia. To clarify this question, rats were subjected to global ischemia by bilateral carotid occlusion and hypotension for 20 minutes, followed by reperfusion for 2 hours (n = 7), 8 hours (n = 7), or 24 hours (n = 7). Cryostat sections were hybridized using antisense oligonucleotide probes (30 dimer). Multiple cell markers were used in immunohistochemical staining to identify the cells expressing IL-lβ and IL- 1R protein. The sham animals (n = 5) showed no or only a weak expression of IL-1R or IL- 1β mRNA. The number of IL-1β mRNA-expressing cells was significantly increased by 2 hours of reperfusion in several brain areas including cortex (12-fold compared with sham) and caudate-putamen (14- fold), and was maximally increased in most hippocampal regions by 8 hours of reperfusion (mean ± SD of positive cells/field versus sham equivalent being 37.9 ± 12.3 versus 4.0 ± 3.3; 30.6 ± 9.0 versus 3.1 ± 2.3; 41.3 ± 17.5 versus 2.9 ± 1.9; in CA1; CA2; CA3/CA4 regions of the hippocampus, respectively). IL-1β mRNA signal was also intensified in the white mater areas. Changes in IL-1R mRNA were seen in the hippocampus (after 2 hours CA1: 16-fold; CA2: 17-fold; DG: 24-fold increase; and CA3/CA4: 10-fold increase after 8 hours), and the expression was prolonged especially in CA1 and CA2 regions up to 24 hours of reperfusion. The major cellular source of IL-1β protein was glia (astrocytes, oligodendrocytes, microglia, and scattered perivascular macrophages/monocytes), while neurons and sporadic microvascular endothelia showed IL-1R immunoreactivity. The data suggest that neurons in discrete areas vulnerable for selective neuronal death, and possibly the vascular endothelium, are target cells for ischemia-induced glial IL-1β production.