An increased diuresis was induced in each of four healthy subjects by intravenous infusion of urea, saline and sodium bicarbonate and by an oral water load. The excretion of a low molecular weight protein (LMW-protein), β2-Microglobulin (β2 µ), was determined by a radioimmunoassay, the urinary concentrations of the protein being in the range 0.007–0,3 µg/ml. The hourly excretion of β2 µ was not significantly influenced by varying loads of water, sodium, and osmoles which suggests that the tubular reabsorption of LMW-proteins is stable under these conditions and that even a moderately increased excretion of such proteins may reflect a disturbed proximal tubular function.