SUMMARYObservations were made on the effect of frusemide given by intravenous injection on the glomerular filtration rate and the water and electrolyte excretion in 12 subjects.The intravenous administration of frusemide caused either no change or slight reduction in the glomerular filtration rate. The basis for the “washout” effect in causing false clearance results is discussed.Changes in free water clearance suggest that the drug has an action on the thick ascending limb of the loop of Henle.Onset of diuresis was detectable in the third minute and the volume and saline diuresis reached a peak as early as the fifth minute after administration of the diuretic. A mean of 22% (range, 4·7% to 34·5%) of the glomerular filtrate, and 14·5% (range, 5·4% to 23%) of the filtered load of sodium, was excreted in the 45 minutes after the intravenous administration of frusemide.