Interpretation of the diuresis renogram is dependent on a full understanding of the factors that contribute to the washout curve, a major determinant being the urinary flow rate induced by the diuretic. Frusemide-induced flow rates were measured in 86 patients over a 30-min period and related to creatinine clearance in 39, 99mTc-DTPA clearance (glomerular filtration rate (GFR)) in 27, and 123I-Hippuran clearance (effective renal plasma flow) in 20. The flow rates 3 to 6 min and 15 to 18 min after administration of the diuretic received specific attention because of their importance to F + 20 and F - 15 diuresis renography respectively. The mean urinary flow rate 15 to 18 min after frusemide was 3.5 ml/min greater than at 3 to 6 min (P less than 0.001), explaining the value of the F - 15 renogram in further evaluating equivocal F + 20 curves. Linear relationships were observed between each parameter of function and the 3 to 6 min and 15 to 18 min periods as follows: creatinine clearance r = 0.66, r = 0.77; 99mTc-DTPA clearance r = 0.66, r = 0.77; 123I-Hippuran clearance r = 0.59, r = 0.71. From regression analysis of the data presented it is possible, with knowledge of total and split function, to predict single-kidney flow rates. It is demonstrated that these may commonly exceed flow rates used for perfusion studies if function is normal, but a single-kidney flow rate of 10 ml/min is unlikely to be attained if the single-kidney GFR is less than 15 ml/min.