Measurements of blood and liver water-soluble vitamins and of their metabolites urinary excretion helped characterize these vitamins metabolism in rats with streptozotocin-induced diabetes. The liver levels of recovered ascorbic acid and total thiamin were virtually the same in diabetic and intact rats. Essential changes of riboflavin metabolism were revealed in diabetes: riboflavin urinary excretion was five times higher than that in intact animals, the blood serum levels being the same in health and disease, whereas liver riboflavin levels in diabetic rats had a trend to lowering. The activities of FAD-dependent hepatic enzymes were the same in both groups. 4-pyridoxylic acid excretion was increased and 1-methylnicotinamide excretion reduced in diabetic rats.