4-hydroxy-3-Methoxyphenylglycol (piperazine salt) is a metabolite of norepinephrine derived in part by deamination of normetanephrine or by O-methylation of dihydroxyphenylglycol after cellular uptake of either intermediate. It is released into the blood and cerebrospinal fluid and has been used as a biochemical index of recent sympathetic nervous system activity.