To investigate the mechanism of the mutagenic action of hydroxylamine in vivo, functional properties have been studied in the RNA polymerase system of cytidine triphosphates modified with hydroxylamine and O-methylhydroxylamine. N4-Hydroxy-6-hydroxyamino- and N4-methoxy-6-methoxyamino-5,6-dihydrocytidine triphosphates exhibit little, if any, activity. N4-Hydroxy- and N4-methoxycytidine triphosphates are functionally active, althugh their activity is somewhat smaller than that of natural nucleoside triphosphates. The compounds substitute for both CTP and UTP and thus exhibit a pronounced amphoteric (dual) functional specificity. In view of the data obtained, the possible genetic consequences of the action of hydroxylamine in vivo and in vitro are discussed. © 1972.