In order to study combined s-dopa and s-a-methyldopa metabolism in brain, we have developed a chemical ionization mass spectrometric method for simuhaneously measuring nanomole quantities of dopamine, a-methyldopamine, norepinephrine, and o-methylnorepinephrine from tissue samples dissected from rat brain. Deuterium-labeled analogs of each of the four catecholamines were synthesized and used as internal standards for quantification. Following addition of the internal standards to the tissue sections, the catecholamines were extracted and separated from amino acids by ion-exchange chromatography. Derivatization with HCI.ethanol and pentafluoropropionic anhydride produced a family of eight derivatives with unique parent masses which were applied without gas chromatographic separation to the direct inlet probe of the mass spectrometer. The mass region containing the derivatives was repetitively scanned, and the individual amines were quantified by comparing their peak heights with the deuterium-labeled internal standards. To validate the solid probe methodin viva, animals were treated with varying combinations of s-dopa and s-a-methyldopa, and their brains were analyzed for amine concentrations. Results from rat hypothalamus, brainstem, and corpus striatum are presented.