We asked whether the likelihood for mice of the C57BL/6J strain to develop glucose intolerance when fed a high-fat diet is related to the increase in circulating levels of leptin or free fatty acids (FFA). We therefore administered a high-fat diet (58% fat) or a control diet (11% fat) for 1.5 years. NMRI mice were used as a more glucose-tolerant control group. After a high-fat diet, the area under the glucose curve following an intraperitoneal glucose challenge (1 g/kg) increased more markedly in C57BL/6J mice (by 42+/-8%) than in NMRI mice (by 21+/-3%, P = 0.007). Plasma levels of insulin, leptin and FFA increased in both strains of mice, whereas plasma glucose levels were elevated after the high-fat diet only in C57BL/6J mice. The slope of the relationship between body weight and plasma leptin was higher in C57BL/6J mice than in NMRI mice. suggesting leptin insensitivity. Circulating leptin correlated to circulating insulin in both strains of mice, whereas plasma FFA correlated to plasma insulin in NMRI mice but not in C57BL/6J mice. These correlations remained significant after adjustment for body weight. The results show that elevated leptin and FFA levels evolve after high-fat feeding in mice, in conjunction with evolvement of glucose intolerance and hyperglycemia.