The influence of instrumentation, algorithm, and the time of blood sampling on whole-body renal clearance as defined by effective renal plasma sampling (ERPF) was investigated. The study involved simultaneous sampling with a partially shielded whole-body counter (WBC) and a gamma camera to assess ERPF. The results obtained using two different analysis algorithms for each modality are compared. Although ERPF as determined by the Oberhausen technique was significantly higher than that obtained using an algorithm developed by the authors, for each algorithm there was no significant difference between the WBC and gamma camera-derived values within the range of 100 to 800 ml/min. Furthermore, positioning of the ROI was not critical for gamma camera-derived ERPF (SD < 1% for multiple crescentic ROIs and approximately 4% for multiple rectangular ROIs). However, the time of blood sampling did appear more important since the average ERPF derived from the 10- and 20-min samples compared with that from the 15- and 25-min samples differed significantly for both analysis algorithms. The methodology which we have validated provides a more precise tool for further investigations of the influence of medication, posture, and exercise stress on renal function.