We compared the properties of monoamine oxidase (MAO) in hamster islets and insulinomas. The tryptamine concentration at which the monoamine oxidase velocity was half maximal (Km) was higher in islets than insulinomas; the maximal velocity (Vmax) of monoamine oxidase was greater in islets than insulinomas. Both islet and insulinoma are more sensitive to inhibition by pargyline than clorgyline. After pargyline administration the monoamine oxidase activity of insulinoma returns faster than activity of islets. This may occur because both synthesis of new MAO in pre-existing cells and formation of new cells occurs in the insulinoma.