Repetitive quantitative renal extraction studies of 99Tcm-MAG 3, performed in nine rats, demonstrated excellent reproducibility of successive measurements. These results are better than with 99Tcm-DTPA, due to the higher renal extraction of the 99Tcm-MAG 3. The % of renal uptake 10 min after the injection was not influenced by the elapsed time between the end of the 99Tcm-MAG 3 preparation and its i.v. administration.