Background: Accurate, precise and straightforward methods for measuring glomerular filtration rate (GFR) and/or renal plasma flow (RPF) are still in demand today. The time-consuming constant infusion technique (CIT) is the gold standard and preferred for research, whereas the simple, but less precise, single injection technique (SIT) is used in clinical settings. This study investigated the use of ^99mTc-DTPA and ^99mTc-MAG₃ by CIT as a measure of renal function. We developed and evaluated a model to balance the primer dose and infusion rate in an attempt to obtain plasma steady state as quickly as possible. Methods: 14 healthy subjects received ^99mTc-DTPA and 6 hypertensive patients received ^99mTc-MAG₃ in a standardized protocol. All participants had an eGFR above 60 ml/min and none had fluid retention. An intravenous primer injection of the relevant tracer was followed by a sustained infusion over 4.5 h with the same radiopharmaceutical. Blood and urine samples were collected at fixed intervals. Results: ^99mTc-DTPA clearance reached steady state after 210 min (plasma clearance 78 ± 18 ml/min, urine clearance 110 ± 28 ml/min), whereas ^99mTc-MAG₃ clearance achieved steady state after 150 min (plasma clearance 212 ± 56 ml/min, urine clearance 233 ± 59 ml/min). Conclusion: Constant infusion technique with fixed primer and infusion rate using ^99mTc-MAG₃ is feasible for research purposes. The longer time for reaching plasma steady state using ^99mTc-DTPA makes CIT with this tracer less optimal. If the primer/sustained balance can be optimized, for example using a priori SIT information, ^99mTc-DTPA as tracer for CIT may also be feasible.