A direct method for measuring the rate of production of neurotransmitter metabolites by the brain of awake monkeys is described. The method utilizes a coupling of a measure of cerebral blood flow with the determination of the difference in concentration of the metabolite under study in arterial and internal jugular bulb blood. A consistent veno-arterial difference for 3-methoxy-4-hydroxyphenethylenglycol (MHPG) has been found. The concentration of MHPG in blood obtained from the right and left venous outflows from brain were not significantly different indicating that blood from either the right or left internal jugular bulb may be used with this method. The rate of production of MHPG by the brain of thw awake monkey is estimated to be 24.1 ng/100 g brain/min. The rate of MHPG production by brain is increased by the administration of piperoxan and decreased by clonidine. Using the experimentally determined rate of production of MHPG by brain and extrapolating to the human it is suggested that a substantial fraction of the total body production of MHPG in man occurs in brain.