1 A sensitive radioenzymatic assay was developed, in which DOPA is enzymatically decarboxylated to dopamine and the latter converted to [3H]-methoxytyramine in the presence of [3H]-S-adenosyl-L-methionine and catechol-o-methyltransferase. 2 The assay was specific for DOPA, and was sensitive to 50 pg/ml. 3 Endogenous DOPA was found to be present in the plasma of eight human volunteers at a concentration of 10.46 +/− 2.42 nmol/l. 4 Simultaneous urine collections in the same subjects showed a free dopamine excretion of 68.88 +/− 17.70 nmol/h. There was significant correlation (P less than 0.01) between plasma DOPA concentration and urine free dopamine excretion (r = 0.84). 5 After the oral administration of 250 mg levodopa, plasma DOPA and urine dopamine both increased by a similar proportion (98 +/− 8.4-fold, and 93.4 +/− 6.9-fold respectively). These compare with an increase in plasma dopamine of only 26 +/− 15-fold (P less than 0.01). 6 Following the orale dose DOPA, the increase in plasma DOPA, but not plasma dopamine, could account for the increase in urine dopamine. The calculated clearance of plasma DOPA by renal decarboxylation to dopamine was 114 +/− 20 ml/min. 7 This is not significantly different from the apparent clearance of endogenous DOPA by renal decarboxylation to dopamine, and suggests that there is adequate renal decarboxylase activity for DOPA to be the precursor for renal dopamine formation.