The MAO activity (tyramine substrate) was measured in 17 different brain regions, the spinal cord, and 13 different organs of four patients with Huntington's disease and three patients with Parkinson's disease. The values were compared with those measured in corresponding tissues from neurologically healthy patients. The serum MAO activities of 12 patients with Huntington's disease were compared with those of 137 normal persons (benzylamine substrate). A comparison of the MAO activities in the brain regions of patients with Huntington's disease, Parkinson's disease, and normal controls showed only minor differences in the cerebral gray matter, corpus callosum, striatum, caudate nucleus, pons, putamen, nucleus niger, dentatus, cerebellar gray matter, and cerebellar white matter. Larger differences in MAO activity were observed in the Ammon's horn, the striatal fundus, and the olfactory region. Slightly increased MAO activities were found in the pallidum of patients with Huntington's disease and Parkinson's disease. In patients with Huntington's disease the MAO activity is considerably increased in the medulla oblongata (25%) and the spinal cord (nearly 50%) as compared to patients with Parkinson's disease and normal controls. The brain MAO activity in the rat remained unchanged after movement anomalies had been induced by application of 2,3-diaminobutyric acid; the neurolathyrogenic substance iminodipropionitrile (IDPN), which causes chloreiform movements in the rat that last for life did not considerably change the MAO activity of the brain. The enzyme values were essentially the same in pancreas, spleen, kidney, adrenal, jejunum, ileum, colon, rectum, and testis of patients and control individuals. The MAO activities in the thyroid glands of patients with Parkinson's disease and Huntington's disease were slightly reduced in comparison with the control group. MAO activity in the liver of patients with Huntington's disease was reduced to approximately 50%. In the heart it was increased by approximately 30%, and that in the lungs was five times higher. A comparison of serum MAO activities in Huntington's disease patients and normal subjects did not reveal any essential differences. The increased MAO values in the sera of female patients with Huntington's disease may be related to hormonal influences.