The precise measurement of glomerular filtration rate (GFR) and renal plasma flow (ERPF) usually requires continuous intravenous administration of adequate substances, with multiple blood and urine analysis, and does not allow measurement of separate renal function. Schlegel et al. and Gates described isotopic methods for the measurement of global and unilateral GFR and ERPF based on the determination by scintillation camera of the fraction of the injected dose ([99mTc]DTPA-[131I]hippuran) present in the kidneys 1-3 min after its administration. These methods require counting of the injected dose and correction for attenuation, but no blood or urine sampling. We have validated these techniques by simultaneous infusion of inulin and PAH in patients with various levels of global renal function (anuric to normal). To better define unilateral renal function we have also studied nine kidneys in patients either nephrectomized or with a nephrostomy enabling unilateral function measurement. A good correlation between inulin or PAH clearances and fractional uptake of [99mTc]DTPA or [131I]hippuran by the kidney was observed. Very good reproducibility of both isotopic techniques was shown. We conclude that determination of the fractional uptake of [99mTc]DTPA and [131I]hippuran between 1 and 3 min allows good and reproducible prediction of global and especially of unilateral kidney function with great rapidity and simplicity, rendering this technique suitable for clinical practice.