Free radical damage has been implicated in the induction of apoptosis in some cells. We investigated whether the status of a cell's oxidant defence system is involved in the signalling pathways triggering apoptosis. We used three unrelated agents, dexamethasone, thapsigargin and gliotoxin to induce apoptosis in thymocytes from 10-day-old BALB/c mice. With all stimuli there was a correlation between the percentage of cells undergoing apoptosis (as measured with propidium iodide DNA staining) and the percentage of cells with lowered [GSH]i. Treatment with either 1 mM reduced glutathione or 10 nM thapsigargin inhibited dexamethasone-induced apoptosis in thymocytes at 6 h, as well as the rise in the percentage of cells with lowered [GSH]i that normally accompanied the onset of apoptosis. Furthermore, following treatment of thymocytes with oxidized glutathione, a normal product of the action of the cell's oxidant defence system, high levels of apoptosis were observed. This suggested that the onset of apoptosis was not simply the result of a loss of GSH from the cytosol. From our evidence we suggest that a decrease in [GSH]i, or an increase in [GSSG]i or perhaps a change in the ratio of [GSH]i to [GSSG]i constitutes a trigger for apoptosis.