AbstractThe preparation of CD3‐labelled 3‐methoxytyramine, normetanephrine and metanephrine from their parent catecholamines is described. The syntheses include N‐maleylation, O‐methylation by dimethylsulfate‐D6, removal of the protecting group by acid hydrolysis and partial purification of the reaction product. Examples of the use of the deuterium labelled O‐methylated catecholamine metabolites as internal standards for the ammonia chemical ionisation mass fragmentographic determination of their naturally occurring analogs in urine