The contribution of free L-DOPA to urinary dopamine (D) was examined by microinjecting 3H-DOPA into either proximal tubules or the peritubular space of innervated and denervated rat kidneys. Recirculation of radioactive material was corrected for by comparison with excretion of 14C inulin included in the injectate. Urine radioactivity was characterized by HPLC. 69.4 ± 2.9% of 3H-DOPA microinjected into proximal tubules appeard in urine from the ipsilateral kidney, 14.6 ± 1.6% of the 3H was recovered as D. After microinjection into the subcapsular peritubular space 4 times as much 3H appeared in the urine from the ipsilateral as from the contralateral kidney. Tubular secretion of total 3H and 3HD was calculated by comparison with 14C inulin excretion. Chronically denervated kidneys secreted 36.0 ± 4.2% of the microinjected 3H; innervated kidneys secreted 35.0 ± 3.0%. 3H-D secretion was 14.5 ± 3.7% and 15.3 ± 2.8% of the total 3H-DOPA microinjected into denervated and innervated kidneys respectively. We estimated that 30% of the urinary free D is derived from circulating free L-DOPA.