The Important norepinephrine metabolite (3-methoxy-4hydroxyphenyl)glycol (MHPG) Is determined In human urine after a two-dimensional HPLC separation. Enzymatically hydrolyzed urine Is first separated on a C18 reversed-phase column using an acetate buffer mobile phase with fluorometrlc or UV absorbance monitoring. The eluent volume containing MHPG (*' 1.0) is collected. A portion of the collected sample Is reinjected on a reversed-phase column and separated with a phosphate buffer-methanol mobile phase. MHPG elutes In 5-6 min (k' 3) and Is detected by use of hydrodynamic amperometry. Recovery of added MHPG Is quantitative and samples are determined with a coefficient of variation of 6-7%.