A new assay for serum dopamine-5-hydroxylase is described. The method utilizes liquid chromatography with amperometric detection. In addition, increased sample throughput Is achieved with the use of column switching. The use and benefits of column switching (split column chromatography) are described in detail. Dopamine is used as the substrate for the enzyme and an Internal standard, -methyldopa, is Included In the procedure. The range of activities found among