accelerated reabsorption in the proximal tubule produced by volume depletion the renal response to chronic depletion of extracellular volume was examined using the techniques of micropuncture depletion of salt and water was produced by administration of furosemide to rats maintained on a sodiumfree diet there was a marked fall in body weight plasma volume and glomerular filtration rate the intrinsic reabsorptive capacity of the proximal tubule measured by the splitdroplet technique was greatly enhanced the acceleration of proximal fluid reabsorption could not be accounted for by changes in filtration rate tubular geometry or aldosterone secretion the halftime of droplet reabsorption in the distal tubule was not altered by sodium depletion an increase in the reabsorption of fluid in the proximal tubule as demonstrated directly in the present experiments provides an explanation for a variety of clinical phenomena associated with volume depletion