Amino acids, such as valine, isoleucine and leucine compete with tyrosine and tryptophan for transport into the brain and might thus affect the central serotonin and catecholamine patterns. Furthermore, the excitatory amino acids glutamic acid, aspartic acid and glycine are known to act on the N-methyl-D-aspartate receptor, which is part of the reward system. Based on these facts, we have explored the role of cerebrospinal fluid (CSF) amino acids in pathological gambling. Concentrations of amino acids were determined in CSF obtained from one female and 11 pathological male gamblers and 11 healthy male controls. In an ANCOVA with best subset regression, pathological male gamblers had higher CSF levels of the excitatory glutamic and aspartic acids, as well as of phenylalanine, isoleucine, citrulline and glycine. A negative contribution of glycine in interaction with the neuraxis distance might mirror a reduced spinal supply or an altered elimination of glycine in pathological gamblers. A decreasing CSF gradient from the first (0-6 ml) to the third (13-18 ml) CSF fraction was found for glutamic acid, glycine, leucine, isoleucine, lysine, ornithine and glutamine in both pathological gamblers and healthy controls. A decreasing gradient was found, however, for aspartic acid and phenylalanine in pathological male gamblers. The altered pattern of CSF amino acids in pathological gamblers might exert an influence on central monoamines as well as on N-methyl-D-aspartate receptor function.