Upregulation of the pro-inflammatory cytokine tumour necrosis factor-α (TNF) occurs rapidly in the brain following ischaemia, although it is unclear whether this represents a neurotoxic or neuroprotective response. We have investigated whether TNF has different actions in the pre- and postischaemic periods in a tissue culture model of cerebral ischaemia. Organotypic hippocampal slice cultures were prepared from 8–10-day-old rats and maintained in vitro for 14 days. Neuronal damage was induced by either 1 h oxygen–glucose deprivation or 3 h exposure to NMDA or the superoxide generator duroquinone, and assessed after 24 h by propidium iodide fluorescence. TNF pretreatment was neuroprotective against both oxygen–glucose deprivation and duroquinone. This effect was associated with an activation of the transcription factor NFκB and upregulation of manganese superoxide dismutase, and was prevented by a free radical scavenger. When addition of TNF was delayed until the postinsult period, an exacerbation of neurotoxicity occurred, which was also prevented by a free radical scavenger. The actions of TNF are determined by whether TNF is present before or after an ischaemia-related insult. Both actions are mediated through the production of free radicals, and the response to TNF is determined by whether a cell is metabolically competent to respond by synthesis of antioxidant defences.