From a series of studies on autopsy brains, the concentration of cystathionine has been found to vary in different areas. In the brains of two homocystinuric children, however, the concentration was found to be extremely low in all the areas examined. In the autopsy tissues of a patient with cystathioninuria the concentrations of cystathionine have been confirmed to be high. These findings are consistent with the view that the biochemical defect in homocystinuria is a deficiency of cystathionine synthetase and that the defect in cystathioninuria is a deficiency of the cleaving enzyme. Investigation of the activity of betaine-homocysteine transmethylase, thetin-homocysteine transmethylase, and glutathione-homocystine transhydrogenase by in vitro techniques revealed no differences between homocystinurics and controls.