1. The distribution of DPN and DPNH pyrophosphatases and DPNase in centrifugally prepared fractions of organs of several species of animals is reported.
2. A DPNH pyrophosphatase was found in the soluble fraction of pigeon and of rabbit liver. This enzyme did not split DPN but accounted for over 50 per cent of the DPNH pyrophosphatase activity of the whole homogenates.
3. All the organs tested, including the pigeon liver and rabbit liver, contained a microsomal pyrophosphatase that attacked both DPNH and DPN. This microsomal enzyme split DPNH faster than DPN in all cases.
4. DPN pyrophosphatase and DPNase activity were generally concentrated in the microsomal fraction of liver, of kidney, and of brain.
5. The DPNase of hamster liver was virtually inactive at pH 7.5 but was optimally active at pH 5.5. Considerable difference was found with respect to pH on the activity of DPNase from organs of different animals.
6. The inhibition of mitochondrial and microsomal DPNH oxidation by nicotinamide was noted during the course of these experiments.
7. The significance of some of the distribution patterns is discussed.