Four groups of gestating female rats were fed diets containing respectively 1, 2, 4, 6 mg of riboflavine per kg. Measurements of the changes in vitamin excretion as related to the dietary intake during various periods of gestation support the hypothesis that riboflavin requirement increases in the last days of gestation. On the other hand, measurements of riboflavin concentration in tissues show that, when the vitamin is given in suboptimal quantity, it is priorly fixed by the foetus. Qualitatively these results are similar to those we obtained in our corresponding study with thiamin. However, the magnitude of the changes in riboflavin urinary excretion or fixation by organs, as related to the dietary intakes, are much more limited as compared to those obtained with thiamin.