Dopamine is a natriuretic hormone that is abundantly synthesized in the kidney and is involved in sodium homeostasis. It is metabolized by monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT) to form 3-methoxytyramine and dihydroxyphenylacetic acid (DOPAC) and finally homovanillic acid (HVA). In order to investigate whether dopamine metabolism is involved in renal sodium regulation, we tested the renal effects of the nitrocatechol entacapone (COMT inhibitor), in comparison with those of the pyridine derivative CGP 28014, in the anaesthetized rat. Entacapone injection resulted in a more than 5-fold increase in sodium excretion, while the renal excretion of dopamine only transiently increased by 20%. DOPAC excretion showed a more than 2-fold increase which persisted throughout the study. Pretreatment with the selective dopamine DA1-receptor antagonist SCH23390 reduced the entacapone-induced natriuretic response by 69%. Glomerular filtration rate (GFR) and mean arterial blood pressure (MAP) remained unchanged. Injection of CGP 28014 did not produce a natriuretic response; nevertheless, both dopamine and DOPAC excretion increased by 78% and more than 2-fold, respectively. GFR and MAP remained unchanged. In conclusion, COMT inhibition using entacapone results in a mainly DA1 receptor mediated natriuresis involving inhibition of tubular transport processes, supporting a role for dopamine metabolism in sodium homeostasis. Although CGP 28014 increases the renal excretion of both dopamine and DOPAC it does not affect renal sodium handling indicating a different mechanism of action.