SUMMARYThe normal range for the urinary excretion of histidine metabolites after an oral loading dose of 15 grams of histidine monohydrochloride was 0–22.3 mg./5 hour urine. The excretion rates of the individual metabolites were formiminoglutamic acid 0–16.6 mg./5 hour urine and urocanic acid 0–9.6 mg./5 hour urine. No significant difference was found between the levels in males and females.The urines of 31 patients were examined for histidine metabolites after oral loading with 15 grams of histidine monohydrochloride. Twenty‐one patients excreted abnormal quantities of Figlu, and eight also excreted an excess of urocanic acid. Urocanic acid may make a significant contribution to the total histidine metabolites excreted.