Preliminary biochemical analyses on plasma, urine, cerebrospinal fluid (CSF) and post mortem brain areas in the rare Rett syndrome indicate no gross disturbance of neurotransmitter function in the periphery. The amino acid pattern, the plasma catecholamines, dopamine, noradrenaline and adrenaline, serotonin in plasma and platelets, and monoamine oxidase (MAO) B-activity in platelets were not different from controls. Urinary metabolites of biogenic amines tended to be increased in the Rett syndrome. Amino acid and noradrenaline concentrations were not changed in lumbar CSF. In a single case of the Rett syndrome, lower values for most amino acids were notable in post-mortem human brain areas and this finding was accompanied by a severe reduction of dopamine, noradrenaline and serotonin, while the metabolite, DOPAC, most times is increased, and HVA and 5-HIAA are decreased. MAO activities, determined in four brain areas, showed no major abnormalities. 3H-spiroperidol binding was significantly below normal in the putamen and 3H tryptamine binding sites in the occipital cortex showed increased binding numbers with no changes in Hill-coefficients. In conclusion, our preliminary data indicate no severe changes in the peripheral neurotransmitter synthesis and turnover, while first post-mortem data indicate severe reduction of biogenic amine synthesis with enhanced turnover and reduced dopaminergic D-2 receptor activity in the advanced stage of a single case of the Rett syndrome.