First clinical results indicated that among all potential 99mTc-complexes the N3S-complex 99mTc MAG3 has the biokinetic properties most similar to o-I-hippurate (OIH). A simultaneous comparison of 99mTc MAG3 and 131I OIH was conducted in a series of 48 patients. Twelve out of 18 transplant patients received simultaneously both radiopharmaceuticals for follow-up scintigraphic studies during the early post-transplantation period. In all of these double tracer studies (n = 90) the renograms obtained with 99mTc MAG3 exhibited qualitatively the same curve-shapes, compared with OIH, in cases of acute graft rejection or acute tubular necrosis. In addition, both agents were administered simultaneously to 46 of these patients and the plasma clearance values were determined under steady state conditions. The clearance data of both radio-pharmaceuticals showed a very good correlation (r = 0.95; P less than or equal to 0.05). The 99mTc MAG3/131I OIH clearance ratio showed a mean value of 0.65. Due to the high plasma protein binding of 99mTc MAG3 it is assumed not to be filtered through the glomerular membrane. Therefore we introduce for the 99mTc MAG3 clearance the term "Tubular Extraction Rate (TER)". Due to the favourable physical characteristics of 99mTc, higher activities can be administered as compared to 131I OIH. This allows the simultaneous examination of the tubular function as well as the renal perfusion. Additionally the kidney morphology is imaged with much better resolution. It is concluded that from the clinical point of view 99mTc MAG3 is a favourable replacement for OIH not only for renal imaging but also for the quantitative determination of tubular function. The acceptance will depend on the quality and on the handling of a commercially available kit formulation.