A method is described for the simultaneous determination of GFR and ERPF within the normal range using a single injection of 51Cr-EDTA and 125I-OIH. Reference values of GFR (range 59-176 ml min-1) and ERPF (268-810 ml min-1) were calculated from bi-exponential analysis of curves based on multiple plasma samples. A subset of the samples (44, 120, 180 and 240 min) was used to estimate GFR by fitting a bi-exponential curve. Regression analysis demonstrated that the reference GFR value could be predicted from this four-sample estimate with a standard error of estimate (SEE) of 2.9 ml min-1, which compared favourably with methods based on mono-exponential analysis (minimum SEE = 6.3 ml min-1) or single samples (minimum SEE = 7.7 ml min-1). The effective volume of distribution of 125I-OIH calculated from the 44 min sample was used to estimate ERPF using a published equation with a SEE of 52 ml min-1. The improved precision of this simplified method for the simultaneous measurement of GFR and ERPF in the normal range will facilitate longitudinal studies in large patient groups in the evaluation of the causative factors of renal disease.