Two groups of rats were bred and maintained for six successive generations on diets containing over 70% by wcight of dried bread. For one group the bread was made from untreated flour and for the other from flour treated with 48 p p m. of chlorine dioxide. For each generation the number of litters produced, the number per litter, the birthweight, the weight of the young a t 7, 14 and 21 days during suckling, the weight a t weaning, the survival rate a t weaning, the post-weaning rate of growth to maturity and food consumption during growth wrre obsrrved. In these respects there was no significant tliffrrence between the two groups in any of the generations or ovcr all generations. The observations as a whole providr no evidence that treatment of flour with chlorine dioxidr at this level produces a faLtor which advrrsrly affects the nutrition of the rat during reproduction and growth.