Volume, osmolality, and concentrations for Na, Cl, and raffinose have been measured as a function of time in standing droplets within rat intermediate and late proximal tubules . Standing droplet reabsorption proceeds without the development of a measurable osmotic difference across the epithelium . After 140 s of tubular exposure, droplet-to-plasma concentration differences are observed for raffinose, Na, and C1 with the observed Na concentration difference, usually referred to as limiting gradient, being -9 mM . It is possible that a smaller or even no limiting difference would be attained with longer exposure times . Previous values measured for the limiting Na concentration in the rat proximal tubule were determined before the attainment of constant concentrations . Assuming that the Na concentration we measured is the limiting value, we estimate that active NaCl transport accounts for a very small fraction,