1. Changes in extracellular water, plasma volume, blood urea and glomerular filtration rate were recorded over intervals of up to 28 days in ten patients requiring diuretic therapy, eight of whom were oedematous and none of whom had obvious renal disease.
2. Diuresis with frusemide was invariably associated with a fall in extracellular water and, in all but one instance, with a fall in plasma volume. It was also associated in every case with a rise in blood urea and fall in glomerular filtration rate.
3. The fall in glomerular filtration rate was observed when the extracellular water and the plasma volume of each patient was still within the ‘normal’ range predicted on the basis of dry weight.
4. Cessation of treatment was followed by a return to initial levels of the plasma volume and extracellular water, blood urea and glomerular filtration rate.
5. When body weight became stable during the administration of frusemide, there was a partial re-expansion of the plasma volume and a tendency for the glomerular filtration rate to rise.
6. The mechanism whereby these changes in renal function are brought about is unknown.