Urinary excretion of m and p nitro, and m amino, but not that of p amino, hippuric acids was increased in the dog by giving NaHCO3 which increased the pH and volume of urine. Tubular excretory transport does not vary among the four substances or with the pH, but at low pH the nitro and m amino compounds undergo tubular reabsorption in accord with their greater lipoid solubility. These results are at variance with conclusions drawn from experiments on renal cortical slices in vitro.