1. Plasma tyrosine and urinary p-hydroxyphenyl lactic acid (PHPLA) and p-hydroxyphenyl acetic acid (PHPAA) were studied in thirty patients with marasmus and twenty normal controls in the same age group. 2. In the control group conventional tyrosyluria was not observed but 30% of the group excreted high levels of PHPAA. In the group with marasmus, plasma tyrosine and urinary PHPLA and PHPAA values were signigificantly higher than the control values. However only 13.3% of the patients were considered to have conventional tyrosyluria and 52.3% were found to excrete high levels of PHPAA. 3. Administration of ascorbic acid resulted in a reduction of PHPLA excretion while it had no effect on PHPAA excretion. 4. It was inferred that (a) tyrosyluria in marasmus is due to the reduced activity of the hepatic enzyme 4-hydroxyphenyl pyruvate: oxygen oxidoreductase (hydroxylating, decarboxylating) (PHPAA-oxidase; EC 1.13.11.27) due to the deficiency of ascorbic acid and (b) high excretion of PHPAA is related to age and nutrition of the child and is unaffected by the administration of ascorbic acid. 5. It was further inferred that urinary excretion of PHPLA is a reliable index of tyrosyluria.