The study included 13 infants under one year complaining of acute gastroenteritis and dehydration who were still in oliguria or anuria 6 hours after rehydration was initiated. They were given a single dose of furosemide at the rate of 1 mg/kg and indices of U/P of urea and osmolarity, ratio urea/plasmatic creatinine, urinary volume, natriuresis and evolution of urea plasmatic figures and of creatinine were determined. Four patients showed no response to the diuretic; all of them died and through clinical and histopathologic evaluation they were classified as having acute renal insufficiency (IRA). The nine patients showing response to the drug with an increase of 5 to 30 times the control figure for urine and natriuresis showed an index U/P of urea of 5.52 +/- 3.82, U/P of osmolarity of 1.32 +/- 09, ratio urea/plasmatic creatinine of 58.7 +/- 19.8 and the figures for urea and creatinine in blood turned normal within 2 to 4 days. This was classed as prerenal azotemia (APR). It is thus concluded that furosemide appears to be a good parameter to make an early differentiation of cases with IRA, but that this measure, the same as the rest of the indices cannot show an absolute value since there are important variations in each individual.