The effect of dietary sucrose and fat in the form of coconut fat (rich in saturated fatty acids) or safflowerseed oil (rich in polyunsaturated fatty acids) was examined on the development of obesity and impaired glucose homeostasis in ob/ob mice. Isoenergetic high sucrose or high fat diets were fed to ob/ob mice from 3-11 weeks of age. Energy intake of mice fed diets rich in fat were similar, and exceeded that attained with the sucrose diet. Body weight gain was greatest in the sucrose-fed mice and least in those fed safflowerseed oil. With the exception of insulin sensitivity which was enhanced with safflowerseed oil, plasma concentrations of glucose and gastric inhibitory polypeptide (GIP), glucose tolerance, intestinal GIP content and the GIP response to oral fat were similar. However, mice fed the high sucrose diet exhibited markedly elevated plasma insulin concentrations and an enhanced pancreatic insulin content. Since the hyperinsulinaemic action of sucrose cannot be attributed to elevated GIP or glucose concentrations, the involvement of other insulin-releasing hormones released from the intestine by sucrose is suggested. The results indicate that the relative amounts of carbohydrate and fat in the diet have an important modulating effect on the development of the ob/ob syndrome. The type of fatty acids in the diet does not appear to be a particularly important determinant for expression of the ob gene.