We describe a simple, rapid, and sensitive procedure for measuring dopamine-β-hydroxylase activity in human blood. The assay is based on the enzymatic conversion of tyramine to octopamine [1-(p-hydroxyphenyl)-2-aminoethanol], which is then oxidized to p-hydroxybenzaldehyde and determined photometrically. Activities are assayed on 2-20 µl of human serum or plasma obtained from fingertip blood samples. The method should prove useful in clinical studies, including the field of pediatrics.