In rats with hereditary Diabetes insipidus (Brattleboro strain) proximal fluid reabsorption was studied during spontaneous variations of GFR. Independent of kidney weight and arterial blood pressure total kidney GFR varied between 0.35 ml/min×gKW and 1.8 ml/min×gKW, while single nephron filtrate showed a variation between 18 nl/min×gKW and 66 nl/min×gKW. End-proximal TF/P inulin of 78 fluid samples did not correlate with filtration rate and averaged 2.38±0.55 S.D. Also, proximal transit time was independent of filtration rate with a mean of 9.7 sec±1.1 S.D. These results imply a direct correlation between mean luminal radius and filtration rate, which was demonstrated by microphotography of the renal surface.