Extracellular nucleotides acting as signaling molecules are inactivated by hydrolysis catalyzed by ecto-nucleotidases. ATP is sequentially degraded via ADP and AMP to adenosine. Enzymes that can be involved in the extracellular hydrolysis chain are ecto-ATP diphosphohydrolase (ecto-apyrase), ecto-ATPase, ecto-ADPase and 5'-nucleotidase. Mammalian ecto-ATP diphosphohydrolase is a member of a family of apyrases sharing four "apyrase conserved regions" that presumably participate in the formation of the catalytic site. We report the presence of ecto-ATP diphosphohydrolase in rat brain and the primary structure of a new mammalian member of the apyrase family. Expression in CHO cells shows that it represents an ecto-ATPase. As revealed by Northern analysis of rat tissues, the ecto-ATPase is co-expressed with ecto-ATP diphosphohydrolase in heart, kidney, spleen, thymus, lung, skeletal muscle and brain. Signals for both ecto-nucleotidases are very weak in liver. mRNAs for both proteins are present in PC12 cells, suggesting that the two nucleotidases may be co-expressed in the same neural cell. Using computer-aided sequence analysis, primary structure and membrane topography are compared with those of other members of the apyrase family.