To investigate the precipitating effects of the westernized diet on diabetes mellitus, glucose tolerance and insulin response to oral glucose load (1.5g/kg body weight) and insulin sensitivity to exogenous insulin (0.2U/kg) were studied in rats fed an experimental diet for 8 weeks. Four experimental diets were used : low fat-no sugar diet (energy ratio of 10% fat, 70% starch, a model of the traditional Japanese diet), high fat-high sugar diet (40% fat, 20% starch, 20% sugar, a model of the westernized diet), low fat-high sugar diet (10% fat, 50% starch, 20% sugar) and high fat-no sugar diet (40% fat, 40% starch). In high fat-high sugar diet rats, the body weight increased more, plasma glucose and insulin levels were higher while fasting and after oral glucose load, and insulin sensitivity was lower than in low fat-no sugar diet rats. Similar impaired glucose tolerance and hyperinsulinemia were present both in low fat-high sugar diet rats and in high fat-no sugar diet rats. However, insulin sensitivity was decreased in low fat-high sugar diet rats, but not in high fat-no sugar diet rats. These data indicate that the high fat-high sugar composition of the westernized diet has some precipitating effects on diabetes mellitus and that the diabetogenic mechanisms of the high fat diet and high sugar diet may be different.