Intravenous administration of maltose (0.4g/kg) in anesthetized dogs produced acute and marked increases in plasma glucose and insulin concentrations to nearly the same extents as the increases after intravenous administration of an equivalent dose of glucose. Plasma maltose disappeared immediately after the injection of maltose. In contrast, intravenous administration of maltose (0.5g/kg) in unanesthetized rabbits failed to produce any significant rises in plasma glucose or insulin concentrations, although plasma maltose concentration increased markedly after maltose injection. This species difference may be ascribed to the difference of plasma maltase activity between these two animals. According to previous reports, maltose is well metabolized in rabbits despite the low plasma maltase activity. The implications of these findings with glucoseinduced insulin secretion are discussed.