In this study we investigated the effects of a high fat diet and exercise on pancreatic -cell function and mass, and its molecular mechanism in 90% pancreatectomized (Px) male rats. The Px diabetic rats were given controlled diets (20 E%, CON) or a high fat diet (45 E%, HF) for 12 weeks. Half of each group was given regular exercise on an uphill treadmill at 20 m/min for 30 min 5 days a week. HF lowered first phase insulin secretion with glucose loading, while exercise training reversed this decrease. However, second phase insulin secretion did not differ among the groups. Exercise increased pancreatic -cell mass. This was resulted from stimulated -cell proliferation and reduced apoptosis, which is associated with potentiated insulin or insulin like growth factor (IGF)-1 signaling through IRS2 induction. Even though HF decreased proliferation and accelerated apoptosis by weakening insulin/IGF-1 signaling due to a reduction of IRS2 protein, HF maintained -cell mass just as much as CON by increasing individual -cell size and neogenesis from precursor cells. Consistent with the results of -cell proliferation, PDX-1 expression increased in the islets of rats in the exercise groups and it was reduced the most in rats fed HF. In conclusion, exercise combined with a moderate fat diet is a good way to maximize -cell function and mass through IRS2 induction in order to alleviate the diabetic condition. This study suggests that dietary fat contents and exercise modulate -cell function and mass to overcome insulin resistance in two different pathways.