Concentrations of catecholamines and their metabolites in plasma entering and exiting mesenteric organs, liver, and kidneys were examined to assess the regional production and metabolism of catecholamines in anesthetized swine. Higher portal venous than arterial plasma concentrations of norepinephrine and its metabolites, 3,4-dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG), and of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), indicated substantial production of both catecholamines by mesenteric organs, representing 45-50% of total body production. Most dopamine was not metabolized to norepinephrine, suggesting a large nonnoradrenergic contribution to total body dopamine production. Concentrations of norepinephrine, DHPG, normetanephrine, and MHPG were lower in plasma exiting than entering the liver, whereas concentrations of the end-product of norepinephrine metabolism, vanillylmandelic acid (VMA), were higher in outflowing than inflowing plasma. Over 94% of the VMA removed by the kidneys was produced in the liver, consistent with the hepatic source of this metabolite. Removal of DOPAC and HVA by the liver exceeded removal by the kidneys or production by mesenteric organs, indicating greater hepatic than renal elimination of these metabolites from the body and underestimation of mesenteric organ dopamine production based on summed spillovers of dopamine metabolites from the liver into the systemic circulation.