Studies demonstrating benzodiazepine-induced reductions in plasma norepinephrine (NE) have assumed that changes in circulating plasma NE closely parallel changes in sympathetic nervous system (SNS) activity and that benzodiazepines suppress SNS outflow. However, decreases in plasma NE could also result from increased removal of NE from plasma via neuronal uptake or tissue metabolism. This study used a tritiated norepinephrine ([3H] NE) isotope dilution technique for measurement of plasma NE kinetics to determine if the fall in plasma NE induced by a single dose of diazepam orally administered to eight psychiatrically-healthy volunteers was due to a fall in plasma NE appearance rate or an increase in plasma NE removal. Diazepam decreased plasma NE appearance, but not clearance, and also decreased plasma epinephrine and mean arterial pressure, memory performance and alertness. Plasma levels of diazepam were correlated with drug effects on memory and alertness but not cardiovascular or SNS effects.