The serum of schizophrenic, alcoholic, and mongoloid patients had no inhibiting effect on the respiratory activity of surviving rat brain. The serum of patients treated with chlorpromazine enhanced the respiratory activity. A slight tendency to elevated values found in recently admitted schizophrenic adults who were not taking chlorpromazine may be due to prior medication before admission. Phenylketonuric serum depressed brain oxidations, though a similar effect was induced by racemic phenylalanine in concentrations above 40 mg.% as well as by other essential amino acids. It has been suggested that the depressant action of various amino acids may be due not to a direct effect of the amino acids, but to aldehyde formation. It has also been known for a long time that a number of toxic amines such as tyramine, phenylethylamine, mescalin, indole and skatol, all inhibit brain oxidations.The observed phenomenon may explain the general depression of brain oxidations found in vivo in phenylketonuria by Himwich and Fazekas. The low cerebral matabolic rate found in mongoloid children in vivo by these authors may be due to the brain pathology associated with the condition.