Preparations of enzymically dispersed rat pancreatic cells hydrolyse externally added nucleoside triphosphates and diphosphates at high rates in the presence of Mg2+ or Ca2+. The lack of response to specific inhibitors and activators differentiates this hydrolytic activity from that of other well-characterized ion-transporting ATPases. Studies based on inactivation of this hydrolytic activity by the covalently reacting, slowly permeating probe diazotized sulphanilic acid indicated that this nucleoside tri- and di-phosphatase is primarily a plasma-membrane ecto-enzyme. It is the major ATPase activity associated with intact cells, homogenates and isolated plasma-membrane fractions. Concanavalin A stimulates this ATPase activity of intact cells and isolated plasma-membrane fractions. The insensitivity of this ATPase activity to univalent ions and inhibitors of pancreatic electrolyte secretion, taken together with the evidence that the active site is externally located, suggests that this enzyme is not directly involved in HCO3- secretion in the pancreas. Its actual function remains unknown.