1. Guinea-pigs on a low, controlled intake of ascorbic acid (0.3 mg/100 g body-weight) were given daily an oral supplement of L-ascorbic acid (AA) or of D-isoascorbic acid (isoAA) (both 1.5 mg/100 g body-weight). The AA was deposited in the tissues, the isoAA was not.2. Intramuscularly administered isoAA was deposited in the tissues but not to the same extent as equivalent amounts of intramuscularly administered AA.3. Paper chromatography confirmed that intramuscularly administered isoAA was deposited in the tissues as isoAA, i.e. there was no conversion into AA.4. It is concluded that in guinea-pigs (a)AA is absorbed from the gastro-intestinal tract much more readily than isoAA and (b)AA is either more readily abstracted from the blood or is more readily retained by the tissues or both, than is isoAA.