While it is clear that there are losses of essential amino acids from brain parenchyma and thus that some influx of amino acids must occur, it is difficult to obtain a quantitative estimate of the influx required. Using radiolabelled amino acids in rats, Dunlop et al. [361–363] found a turnover rate for the protein content of rat brains to be about 0.6% h−1. Using a protein content of about 100 mg for each gram of brain and the molecular weight of an average amino acid, perhaps 125 Da, that corresponds to a rate of incorporation of amino acids of about 80 nmol g−1 min−1. Similarly amino groups required for de novo synthesis of glutamate amount to about 100 nmol g−1 min−1 (see legend to Fig. 17).