The fate of catecholamines released in rat kidneys was examined by microinjecting tritiated norepinephrine (NE), normetanephrine (NMN), dopamine (D) and 3-methoxytyramine (3MT) into proximal and distal tubule lumina and peritubular capillaries. [14C] Inulin included in the injectate served as a reference standard. After injection of NE, D, NMN and 3MT at 0.1 to 4.0 pg/min for 4 to 10 min into proximal tubules, 87 to 94% was recovered in the urine of the injected kidney. Less than 10% of the amines was chemically altered during passage through the nephron. After injection into capillaries, percentages secreted in the first passage through the kidney were: D, 25 +/- 5; 3MT, 47 +/- 3; NE, 13 +/- 2; and NMN, 4 +/- 3. After D capillary injections, excretion of 3MT and D from the microinjected was twice that from the contralateral kidney. Acidic metabolites, homovanillic acid and dihydroxyphenylacetic acid were excreted equally by both kidneys. There was no appreciable difference in the products excreted from left and right kidneys after NE injections. In summary, D released into the peritubular space is converted to 3MT and both compounds are secreted avidly. NE released into the peritubular circulation is converted to NMN and both compounds are secreted but to a much smaller extent than occurs with D and 3MT.