A sensitive fluorimetric method for the assay of dopamine β-hydroxylase in rat plasma is described. Octopamine, formed enzymatically from the substrate tyramine, is separated by chromatography on a Dowex 50W-X4 column and oxidized with periodate to p-hydroxybenzaldehyde, which is then quantitated by means of the fluorimetric method for selective determination of aromatic aldehydes with 2, 2'-dithiobis (1-aminonaphthalene). The method is readily performed with good precision and is suitable for the assay of many samples simultaneously.