An adult male fed by parenteral alimentation for 48 days had successive intakes of a complete mixture of essential amino acids, and then lysine- and histidine deletion to test for the essentially of histidine. After 22 days of histidine-free-intake, 15NH4Cl was ingested orally with subsequent measurement of the time patterns for the appearance of 15N in total plasma proteins, globin from hemoglobin and urine. The 15N was incorporated into glutamate, aspartate, tyrosine, and histidine, but not lysine, of plasma proteins and globin. Urinary histidine and Ntau-methylhistidine, but not lysine, also contained 15N. Degradation of several histidine samples by two methods and two Ntau-methylhistidine samples demonstrated the presence of 15N in both the alpha-amino nitrogen and the imidazole ring. The incorporation of 15N into the ring indicates limited synthesis of histidine in an adult man under the above stated conditions.