Abstract: An improvement in the method for assaying normetanephrine and octopamine. A procedure is used in which the phenylethanolamine in a sample is enzymatically transmethylated in an incubation mixture of the phenylethanolamine, a compound containing a transferable tritiated methyl group and a transfer enzyme, to form a tritiated N-methyl derivative, and the radioactivity of the derivative is measured. In accordance with the present improvement, incubation of the mixture is conducted at a pH of at least 8.6, preferably in the range of 8.8-9.3. Prior to incubation, as appropriate, the sample may be deproteinized and the phenylethanolamine concentrated.