The potentially neurotrophic cytokine transforming growth factor‐β₁ (TGF‐β₁) is locally expressed following human stroke and experimental ischemic lesions, but the cellular source(s) and profile of induction have so far not been established in experimental focal cerebral ischemia. This study presents the time course and a cellular localization of TGF‐β₁ mRNA, visualized by in situ hybridization combined with immunohistochemical staining for microglia, macrophages, or astrocytes, on brain sections from adult spontaneously hypertensive rats subjected to transient proximal occlusion of their middle cerebral artery. Six hours after ischemia, an early and transient neuronal and microglial expression of TGF‐β₁ mRNA was observed in the extraischemic cingulate and frontal cortices. Both early and protracted expression of TGF‐β₁ mRNA in the caudate‐putamen and neocortical infarcts and in the caudate‐putamen penumbra colocalized with OX42/ ED1‐immunoreactive microglia and macrophages, whereas TGF‐β₁ mRNA in the neocortical penumbra colocalized with OX42/ ED1‐immunoreactive cells of a microglial morphology. No astrocytes were double‐labeled. The number of TGF‐β₁ mRNA‐expressing microglia and macrophages increased strongly during the first week. Thereafter, TGF‐β₁ mRNA became increasingly restricted to the neocortical penumbra (3 weeks), and after 3 months it was confined to activated microglia in the anterior commisure. Our data establish activated microglia and macrophages as the major source of TGF‐β₁ mRNA following experimental focal cerebral ischemia. Consequently, TGF‐β₁‐mediated functions may be exerted by microglia both in the early degenerative phase, and later in combination with blood‐borne macrophages, in the remodeling and healing phase after focal cerebral ischemia. GLIA 24:437–448, 1998. © 1998 Wiley‐Liss, Inc.