We studied the synthesis of 57Co-EDTA (Eγ = 122 keV), its biodistribution in Wistar rats and its blood and urinary elimination compared with that of 51Cr-EDTA. We added 6 μmol EDTA diluted in 3-5 ml isotonic phosphate buffer (Na2HPO4) to a commercial 57CoCl2 radioactive tracer solution. The incubation period was 15 min. Quality control was performed using TLC and HPLC. Six healthy Wistar rats underwent 57Co-EDTA renography for 30 min. In one rat, additional TLC and HPLC was performed on blood (one sample only) and urine samples (n = 3) obtained 30 min, 30 min, 2 h and 4 h following injection of 18.5 MBq 57Co-EDTA and 51Cr-EDTA respectively. Radioisotope quantification was done by means of a germanium detector. 57Co was chelated to EDTA at high yield (Kstab = 10E36). No free or protein-bound 57Co was found. The ratio of 51Cr-EDTA to 57Co-EDTA remained constant (P = 0.133, n = 4). 57Co-EDTA was rapidly cleared from the blood pool (heart), and prompt and high target-to-background ratios for both kidneys were obtained (mean = 8.4, range = 7-12). At the end of the acquisition, activity remaining in the body excluding kidney and bladder was 45±5.2%. No specific activity uptake was noted in any other organ or tissue. We conclude that 57Co-EDTA is a promising radioligand for simultaneous clearance and separate renal function estimation. Its preparation is straightforward and, in rats, no free or protein-bound 57Co was found.