The genetically obese mouse (C57BL/6J ob/ob) is a commonly used model of non-insulin-dependent diabetes mellitus. However, our studies demonstrate that, while the animal is significantly hyperinsulinemic, it in fact does not show consistent hyperglycemia in the resting state. During stress, both obese animals and their lean littermates become hyperglycemic, but the magnitude of the hyperglycemia is exaggerated in the obese mice. Obese animals also show an exaggerated plasma glucose increase in response to epinephrine injection. This increase in plasma glucose is accompanied by a decrease in plasma insulin in response to both stress and epinephrine. Our findings suggest that environmental stimuli influence the expression of diabetes in the C57BL/6J obese mouse and therefore must be considered in studies of this animal model of diabetes.