Methylglyoxal is a spontaneous product of glucose metabolism which is known to have cytotoxic actions and to be present in raised concentrations in hyperglycaemia. It could therefore play an important role in glucose toxicity. We have investigated the cytotoxic effects of methylglyoxal on insulin-secreting cells, which are particularly sensitive to glucose toxicity. Methylglyoxal caused a concentration-dependent increase in the number of apoptotic RINm5F cells within 4-6 hours. A similar effect was observed with rat pancreatic beta-cells. tert-butylglyoxal, which is a poor substrate for the glyoxalase pathway, exerted a similar, though more potent apoptotic action. Dexamethasone and NaF were also found to induce apoptosis in RINm5F cells. Flow cytometric analysis suggested a degree of necrosis in addition to apoptosis resulting from treatment with methylglyoxal. The cytotoxic effect of methylglyoxal could contribute towards glucose toxicity in insulin-secreting cells.