Chronic toxicity of riboflavin tetrabutyrate was examined using male albino rats of Wistar strain as test animals. They were administered compulsorily with 2.0 g/day/kg of body weight, 1.0 g/day/kg, or 0.5 g/day/kg of riboflavin tetrabutyrate. The body weight gains of these animals were the same with those of control animals. After 6 months' feeding, the measurement of weight of each organ, hematological examination on blood, and histological investigation on organs and tissues were performed. No difference was found between animals fed with riboflavin tetrabutyrate and controls, indicating that this vitamin derivative has no chronic toxicity.