The role of extraneuronal uptake in terminating the actions of catecholamines has been difficult to evaluate in vivo, largely because of lack of suitable inhibitors. The compound, 1,1'-diisopropyl-2,4'-cyanine iodide or disprocynium24 (D24), is a novel inhibitor of extraneuronal uptake with a high degree of potency in vitro. This study examined the actions of D24 on the inactivation and metabolism of circulating noradrenaline and adrenaline in conscious rats. Animals received i.v. infusions of 3H-labelled noradrenaline and adrenaline, and their extraneuronal O-methylated metabolites, normetanephrine and metanephrine. Plasma concentrations of endogeneous and 3H-labelled catecholamines and metanephrines were measured before and after D24. D24 caused large increases in plasma concentrations of noradrenaline and adrenaline, effects due to both decreases in their plasma clearances and increases in their rates of release into plasma. Plasma concentrations of normetanephrine and metanephrine also increased due to their decreased clearance from plasma. Increased release of normetanephrine into plasma did not contribute to increased plasma concentrations of normetanephrine. In fact, the contribution of extraneuronal O-methylation to noradrenaline clearance decreased substantially after D24. The data indicate that D24 is a potent inhibitor of the extraneuronal catecholamine transporter in vivo and that this process contributes importantly to the removal of circulating catecholamines and their O-methylated amine metabolites. Increased release of noradrenaline into plasma may reflect an increase in the proportion of transmitter that escapes from sites of release into the circulation. However, increased adrenaline release indicates that the drug also causes sympathoadrenal activation.