Aspartame is a widely used artificial sweetener added to many soft beverages and its usage is increasing in health-conscious societies. Upon ingestion, this artificial sweetener produces methanol as a metabolite. In order to examine the possibility of aspartame toxicity, the effects of methanol and its metabolites (formaldehyde and formate) on dissociated rat thymocytes were studied by flow cytometry. While methanol and formate did not affect cell viability in the physiological pH range, formaldehyde at 1-3 mmol/L started to induce cell death. Further increase in formaldehyde concentration produced a dose-dependent decrease in cell viability. Formaldehyde at 1 mmol/L or more greatly reduced cellular content of glutathione, possibly increasing cell vulnerability to oxidative stress. Furthermore, formaldehyde at 3 mmol/L or more significantly increased intracellular concentration of Ca2+ ([Ca2+]i) in a dose-dependent manner. Threshold concentrations of formaldehyde, a metabolite of methanol, that affected the [Ca2+]i and cellular glutathione content were slightly higher than the blood concentrations of methanol previously reported in subjects administered abuse doses of aspartame. It is suggested that aspartame at abuse doses is harmless to humans.