MANNARINO et al1 found that following the injection of 14carbon labeled norepinephrine (NE) (14C-NE) into the lateral ventricle of a cat, the principle labeled o-methylated deaminated metabolite recovered in the cerebrum and cerebellum was 3-methoxy-4-hydroxy phenylglycol (MHPG) and, that in a three-hour pool of urine, over 50% of the recovered radioactivity was present as a conjugate of MHPG. However, due to the fact that with this method some of the injected isotope enters the body pool,2 the exact contribution of peripheral stores of norepinephrine (NE) in urinary metabolites could not be given with certainty. Maas and Landis3,4 using a double-isotope technique in the dog, differentiated the metabolism of NE by brain and body pools and found, in agreement with Mannarino et al, that the major metabolite of brain NE excreted in urine, throughout the 12-hour experimental period, was MHPG.