A method for simultaneous quantification of plasma homovanillic acid and 5-hydroxyindoleacetic acid has been developed, permitting more efficient neurochemical examinations of these often interrelated biogenic amine systems. Zinc sulphate and sodium hydroxide solutions were used for precipitating the protein in plasma prior to injection on the column. This technique allows for cleaner chromatography, greater sensitivity and high precision. The method uses high performance liquid chromatographic separations of these compounds on C18 reversed phase columns with electrochemical detection. The detailed results from controls and untreated parasuicide patients are given.