DCTQ (SK&F 64139) is a potent inhibitor of both adrenal and central nervous system (CNS) phenylethanolamine N-methyltransferase (PNMT). In animal studies, a plasma level of 0.35 µg/ml was associated with 50% inhibition of both adrenal and central PNMT. We performed single-dose phase I studies with DCTQ in man. Plasma drug levels up to 6.26 µg/ml were readily obtained. There were few subjective and no objective clinical changes. DCTQ did not alter blood pressure or cause CNS symptoms in man. Furthermore, resting plasma and urinary catecholamines did not change after DCTQ. The study suggests that acute inhibition of PNMT under resting conditions is without significant clinical effect.