Summary—Minute‐by‐minute urinary flow rates before and after the intravenous injection of 40 mg frusemide were investigated in 29 well hydrated adult patients with varying creatinine clearances. Total urinary flow rates 3 to 6 min after frusemide ranged from 1 to 42 ml/min (mean 19). At 15 to 18 min after the diuretic, flow rates ranged from 8 to 46 ml/min (mean 23). Flow rates were proportional to creatinine clearance (r = 0.73; r = 0.8) and decreased with falling clearance, although significant diuresis (4 ml/min/kidney) was achieved down to single kidney clearances of 10 ml/min. Where single kidney clearances were less than 31 ml/min, single kidney flow rates of 10 ml/min or more were not guaranteed. Below this clearance, diuresis renography remains completely reliable and unequivocal in the majority of cases. However, perfusion pressure flow studies will not necessarily be urodynamically comparable and it should not be expected that the results of the two tests will agree in individual cases of upper tract dilatation. Recommended timing for frusemide administration should be 20 min into the test, or 15 min before radiopharmaceutical inje