We report body weights (BW) and blood and serum analyses for 6 fully mature and 8 not-yet-mature captive plains viscachas before and 3, 6 and 9 months after switching from a low-fiber, high-energy diet to a high-fiber, low-energy diet. Initially, body weights, serum glucose, fructosamine and cholesterol levels were above the reference range in the fully mature animals. Furthermore, 4 of these animals had bilateral cataracts. After the diet change, these parameters dropped into the reference range. However, 9 months later, a slightly increased BW became evident again. The findings are consistent with a type II diabetes mellitus and underline the importance of dietary prevention.