1. We estimated the extent to which circulating dopa (3,4-dihydroxyphenylalanine) is the source of urinary dopamine (DA; 3,4-dihydroxyphenethylamine). Tritiated dopa ([3H]dopa) was infused for 90 min into the left renal artery of seven anaesthetized foxhounds, and levels of labelled and unlabelled dopa and DA were measured in the ureteral urine and in the femoral arterial and left renal venous plasma. 2. Only a small percentage of [3H]dopa delivered to the kidneys was excreted as [3H]DA (0.59% from the left kidney, 0.68% from the right); however, the arterial concentration of endogenous dopa (1220 pg/ml) and the renal plasma flows (144 and 141 ml/min by p-aminohippurate clearances) were such that all of the urinary excretion of endogenous DA (about 1 ng/min from each kidney) could be accounted for by uptake and decarboxylation of circulating endogenous dopa. 3. Plasma dopa is the main source of urinary DA.