Two separate lines--diabetic and partially diabetes-resistant--have been isolated from the sand rat (Psammomys obesus), each with different growth characteristics in response to diets of varying digestible caloric densities (high energy, HE, 2.93 kcal/g, or low energy, LE, 2.38 kcal/g). Over a two week period all animals consumed similar quantities (c. 125 g) irrespective of the diet consumed. Weight gains were as follows: diabetic line on HE diet - 59.7 g, on LE - 46.2 g; non-diabetic animals from the diabetes-resistant line on HE - 44 g. Only animals from the diabetic line, fed the HE diet, developed hyperinsulinemia, obesity and diabetes. The energy cost of weight gain for the diabetic line fed either HE or LE diets was 6.0 - 6.3 kcal/g whereas for the diabetes-resistant line on the HE diet, the cost of growth was 50% higher at 9.3 kcal/g. These differences could be due either to alterations in the content of tissue laid down or to differences in energy expenditure. It has already been established that diet-induced obesity and diabetes develop in the diabetic line with features typical of insulin resistance in the metabolism of the pancreas, liver and peripheral tissues. Some of the animals of the diabetes-resistant line may also develop diabetes over a long time period and go through a phase of transient hyperinsulinemia-normoglycemia. This may represent an intermediate stage in the development of the diabetic syndrome and serve as a model of type 2 diabetes in man.