Dogs were fed for 30-111 days on a deficient diet, which was low in protein and riboflavin, and high in carbohydrate. The composition of the deficient diet was similar to that which has customarily been taken by the inhabitants in Hirosaki area where nutritional dystrophy (so-called Shibi-Gatchaki) is prevalent. The riboflavin concentration in portal blood was estimated prior to, one hour, three hours, and five hours after the injection of riboflavin into the stomach of dogs. An increase in riboflavin, especially the esterified form, in the portal blood was markedly smaller in the malnourished dogs than in the control. The result showed that the intestinal mucosa of malnourished dogs was inferior to the control in the phosphorylating activity for the phosphorylation of riboflavin.