Absorption, urinary excretion and incorporation of riboflavin-tetrabutyrate into FAD, FMN and riboflavin were investigated by using radioactive riboflavin-tetrabutyrate in which carbon in the 2 position of alloxazine was labeled. After oral administration, riboflavin tetrabutyrate and others, identified as rivoflavin tri-, di-, and mono-butyrate by paper chromatography were found in the serum of portal vein as well as in the liver, the kidneys and the skin. Incorporations of ^<14>C into FAD, FMN and riboflavin in the liver after oral and subcutaneous administration of equivalent amount of riboflavin-^<14>C, were found to be similar. The fact that relatively large amounts of not identified radioactive metabolites were found following oral administration of riboflavin tetrabutyrate-^<14>C suggests some different metabolism other than to be hydrolysed to riboflavin in the tissues.