Free water reabsorption (Twc) was measured in conscious dogs during a long-term, reasonably steady-state osmotic diuresis induced either by sodium salts or by urea. After 4 to 8 hr the original solute was partially replaced by urea or by sodium salts. Both the replacement of sodium salts by urea and the converse procedure were associated with an augmentation in Twc. During sodium diuresis Twc always fell during the first 2 or 3 hr, a change which appeared unrelated to glomerular filtration rate (GFR) or to osmolar clearance. During the course of urea diuresis Twc steadily increased and showed a positive correlation with GFR. A possible basis for such a correlation is discussed.
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Maude et al. (1963) studied this question.