The requirements of the brain for some amino acids.

1. A constant specific activity of radioactively labelled amino acids was maintained in the circulation by means of a new technique devised for this purpose. This has made it possible to measure accurately the entry rates of amino acids into the brain in vivo. 2. The rates of entry into the brain of seven nutritionally non-essential amino acids were measured. 3. Glycine and proline enter the brain relatively slowly, at rates comparable to those of amino acids which are not normally found in the blood. Thus their entry is due mainly if not entirely to passive diffusion. 4. Serine (which is used by the brain to make glycine) and alanine (which is used to make glutamate and aspartate) enter the brain as rapidly as the essential amino acids and thus, although not essential for the body as a whole, appear to be essential for the brain. 5. It is suggested that those amino acids that the brain is able to synthesize have low rates of entry, even though they are present at high concentrations in the plasma, but that the transport systems for those amino acids that are not synthesized in the brain ensure rapid entry at rates that are related to the rates of cerebral utilization.