Infusion of an acidic amino acid, L-aspartate, to 10 volunteers resulted in transient, significant increases in urinary excretion of the major urinary trypsin inhibitor (p < 0.002) and β<sub>2</sub>-microglobulin (p < 0.02). Simultaneously with the proteinuria, urinary pH rose significantly (p < 0.02). These changes appeared following the infusion and after the excretion of L-aspartate had reached the preinfusion level. Albumin excretion was unchanged indicating that the proteinuria was due to a decreased tubular reabsorption. The mechanism for the reversible tubular proteinuria is unknown. A simple pH effect due to alkalization of the urine was excluded, as NaHCO<sub>3</sub> infusion was not followed by an increase in the excretion of the major urinary trypsin inhibitor and β<sub>2</sub>-microglobulin.