When 40%> or more of the anhydroglucose units of starch are oxidized to dialdehydes, the material is made toxic to rats as manifested by poor growth, increased mortality and testicular damage. No acute toxicity was noted. No toxicity was shown when 2$ or less of the units were oxidized, even at levels of 20%, of the diet. Starch further oxidized to contain 5$ of the glucose units in the dicarboxyl form produced no toxic symptoms with as much as 20$ of the diet consisting of this modified starch, but food intake was increased and utilization decreased, perhaps due to faulty digestion of the dicarboxyl starch.