We estimated nucleotide pyrophosphatase and phosphodiesterase I activities in human and rat organs and in body fluids from man and dog. The highest organ activities were found in epididymis, kidney, liver, and intestine. In body fluids, the activity was highest in seminal plasma, followed by intestinal lymph, serum, heart lymph, cerebrospinal fluid, milk, and urine. The ratio nucleotide pyrophosphatase/phosphodiesterase I and the urea resistance of phosphodiesterase I differed among human organs, body fluids, and blood cells. Different isoenzymes probably exist. The activities in serum share several properties with those in several organs--e.g. pH-optimum 9.6-9.8, dependency on Zn2+, and the effects of inhibitors. Phosphodiesterase I in erythrocytes, which has not been described previously, differs from enzyme from other sources by lower pH optimum (8.5), dependency on Mg2+, inhibition by Zn2+, and stimulation by dithiothreitol.