Studies were made of d-amino acid oxidase activity in mouse kidney by a simplified and rapid assay procedure. Feeding dl-alanine resulted in a decreased activity per gram of kidney, but growth of a subcutaneous solid ascites tumor did not affect the activity. Oxidase activity was found in germ-free mice. The distribution of d-alanine-C14 in nephrectomized mice was determined in normal and tumor-bearing mice. No metabolic conversions or incorporation into liver or tumor protein were observed, although the liver, lungs, and spleen seemed to concentrate the d-alanine to a small extent. An attempt was made to measure d-amino acid oxidase activity in vivo by the injection of d-alanine-C14 and observation of the radioactivity in the respiratory CO2.