Levels of 5-hydroxyindoleacetic acid, 5-HIAA, the main serotoninergic metabolite in the central nervous system, were investigated in the cerebrospinal fluid, CSF, of male alcoholics and healthy controls. During intoxication, 5-HIAA correlated positively to blood ethanol concentration and levels gradually decreased by length of abstinence. In patients investigated after three months of alcohol abstinence, subnormal levels were noted. Assuming that the level of 5-HIAA in the CSF reflects central serotoninergic activity, the results support the assumption of subnormal activity in abstinent alcoholics and an activation during abuse.