Understanding the role of tumor necrosis factor (TNF) in the life-death balance of ischemically injured neurons demands insight into the cellular synthesis of TNF, especially in the acute phase after induction of ischemia. Here, using approximated stereological methods and quantitative reverse transcription (RT) real-time polymerase chain reaction (PCR) analysis, the cellular synthesis of TNF from 30 mins to 10 days after induction of focal cerebral ischemia in mice was investigated. Reverse transcription real-time PCR analysis showed that TNF mRNA increased 2- to 3-fold within 1 hour after induction of ischemia. A significant 8-fold increase was observed at 4 hours when faintly labelled TNF mRNA-expressing and TNF immunoreactive microglial-like cells were easily identifiable in the peri-infarct and infarct. By 6 hours, TNF synthesizing cells were identified as Mac-1 immunopositive, glial fibrillary acidic protein immunonegative microglia-macrophages. The level of TNF mRNA and the numbers of TNF mRNA-expressing microglia-macrophages peaked at 12 hours, and the number of TNF immunoreactive cells at 24 hours. Neuronal TNF mRNA and TNF protein levels remained at constant, very low, levels. The data suggest that the pathophysiologically important TNF, produced in the acute phase from mins to 6 hours after an ischemic attack in mice, is synthesized by microglia-macrophages.