1. The rate of removal from the circulation of an intravenous lactic acid load has been studied in conscious rats, previously subjected either to bilateral nephrectomy or to a sham operation.
2. In rats with normal blood pH, the apparent contribution of the kidneys to removal of the lactic acid load is 30%; less than 12% of the renal contribution is attributable to urinary excretion.
3. In bilaterally nephrectomized rats made acidotic by administration of ammonium chloride, the rate of removal of a half-neutralized lactic acid load is progressively decreased with increasing severity of acidosis. No such effect is seen in sham-operated animals.
4. An increase in the ability of the kidney to remove lactate during acidosis compensates for approximately half of the simultaneous fall in the capacity of the remainder of the body for lactate assimilation.
5. Basal blood lactate concentrations fall in the presence of metabolic acidosis.