N-Aminated nucleoside complexes of cis-dichloroplatinum(II) were synthesized, and their antitumor activities against L1210 cells in mice and in vitro were studied. While the native nucleosides failed to show any antitumor activity, the complexes exhibited high antitumor activity and had no nephrotoxicity in mice. Studies on their mode of action in vitro indicated that the ligands played characteristic roles in the appearance of antitumor activity; ribonucleoside complexes caused the inhibition of RNA synthesis, and deoxyribo- or arabinonucleoside complexes caused the inhibition of DNA synthesis.