Abstract: A method for determining the norepinephrine activity of a human brain may provide information useful to the diagnosis and treatment of affective disorders. In such a method, the norepinephrine metabolite, 3-methoxy-4-hydroxyphenylglycol, produced by the brain can be measured separately from that produced by the peripheral nervous system. A sample is taken for measuring the total body levels of the norepinephrine metabolites, 3-methoxy-4-hydroxyphenylglycol and vanillylmandelic acid. A dose of isotopically labelled norepinephrine is administered to the body of the subject. Subsequent to the administration of the labelled norepinephrine, samples are collected for measuring the total body levels of isotopically labelled metabolites, 3-methoxy-4-hydroxyphenylglycol and vanillylmandelic acid. Since vanillylmandelic acid is metabolized from norepinephrine substantially entirely in the peripheral nervous system, the brain's contribution of 3-methoxy-4-hydroxyphenylglycol can be determined.