After the oral administration of 5,6-O-isopropylidene-ascorbic acid (IAsA) and 5,6-O-isopropylidene-ascorbic acid 2-sulfate (IAsS), vitamin C levels in blood, liver, kidney, brain and muscle were determined to investigate the con-version of these two compounds into vitamin C in rainbow trout (body weight 160〜190g). Total vitamin C contents of these biological samples were deter-mined by the 2,4-dinitrophenylhydrazine method [J. Biol. Chem., 168, 197 (1947), Vitamins, 43, 210 (1971)]. Vitamin C levels in blood and these tissues were significantly increased in the IAsA-administered group, but slightly in the IAsS-administered group. Therefore, IAsA may be more effective than IAsS as dietary vitamin C source for rainbow trout.