Chronic stress induces abnormal mental state and behavior, and can be a risk factor for mental disorders. Although it is reported that l-tyrosine, an amino acid that is a precursor of catecholamine synthesis, alleviated the change of cognition and behavior induced by acute stress, knowledge about its effects on chronic stress is limited. In the present study, the effects of dietary l-tyrosine on behavioral alteration induced by chronic stress were investigated by employing a social isolation stress model in mice. Social isolation stress increased locomotor activity in both the home cage and open field. These increases of locomotor activity were suppressed by dietary l-tyrosine. Moreover, l-tyrosine increased both the concentration and turnover rate of norepinephrine metabolites. These findings partly suggest the availability of dietary l-tyrosine for psychic dysfunctions induced by chronic stress.