The amino acid content of the central nervous system (CNS) is controlled by the blood-brain barrier. Despite a constant exchange of amino acids in both directions, this interface mediates a net uptake from blood plasma into cerebrospinal fluid (CSF). As in most other cells, amino acids in the brain have their roles as constituents of protein, metabolic precursors, and intermediates in energy and nitrogen metabolism; furthermore these compounds are involved in osmoregulation The system in which these functions are most fully understood is probably the Ehrlich ascites tumor cell (Johnstone, 1979) In the brain, some amino acids clearly have an additional function; that of neurotransmitter, i.e., a chemical messenger that bridges the synaptic clefts between neural membranes, thus mediating interneuronal signaling. Glutamate and aspartate are established excitatory transmitters, whereas this role for cysteate and cysteine sulfinate is putative. Inhibitory transmitters are γ-aminobutyric acid (GABA), glycine (in the spinal cord), and taurine (regarded as a more general neuromodulator) Glutamine is not a neurotransmitter, but an important transport vehicle to transfer neurotransmitters in an inactive form between cells. This compound is, therefore, also to be considered in the present context.