Abstract: Cerebral nitrogen metabolism was studied in 29 healthy nonobese volunteers by means of a catheterization technique. Arterial levels and arterial-jugular venous (A-JV) concentration differences for amino acids, urea, ammonia, 5-oxoproline, glucose, and oxygen were measured in the basal, postabsorptive state and during an intravenous infusion of a commercial amino acid solution. In the basal state positive A-JV differences, indicating a net brain uptake, were noted for 12 of 22 amino acids as well as for ammonia. There was no significant net exchange for urea or for 5-oxoproline. During amino acid infusion, resulting in a 150–300% rise in arterial amino acid levels, the brain uptake of isoleucine, leucine, and tyrosine increased significantly, and a similar tendency was seen for most other amino acids. The infusion was accompanied by a 100% rise in arterial ammonia levels and a 10% increase in urea concentration. For ammonia the small positive A-JV difference in the basal state became markedly greater during amino acid infusion, whereas no significant alteration was noted for urea exchange across the brain. The A-JV differences for glucose and oxygen were positive in the basal state and unchanged during the infusion. The present findings demonstrate that in the basal state (a) there is a significant net brain uptake of most amino acids; (b) no single amino acid, urea, or 5-oxoproline is released from the brain; and (c) ammonia uptake occurs both in this state and during an amino acid infusion. It is concluded that brain uptake of amino acids accounts for a quantitatively significant proportion of the total amino acid turnover in the basal state but the nature of the nitrogenous compound(s) released from the brain to achieve nitrogen balance remains to be determined. Eriksson L. S. et al. Nitrogen metabolism of the human brain.