Abstract: Pyridopyrimidine nucleotide derivatives expressed by formula (I), which are fluorescent and capable of forming base pairing with guanine or adenine, and oligo- or polynucleotide which have at least one of these derivatives in their molecules or at their molecular terminals: ##STR1## wherein X.sub.1 and X.sub.1 respectively represent ##STR2## (in which n indicates an integer of 0, 1, 2, or 3; however, in no case X.sub.1 and Y.sub.1 both indicate n=O); Z.sub.1 indicates a hydrogen atom or ##STR3## (in which n indicates an integer of 1, 2, or 3); W.sub.1 indicates a hydrogen atom or a hydroxyl group. As for R.sub.1 and R.sub.2, in case where R.sub.1 is an amino group or halogen, R.sub.2 indicates a single bond between the carbon atom at the 7-position and the nitrogen atom at the 8-position, and in case where R.sub.2 is a hydrogen atom or an lower alkyl group, R.sub.1 indicates a carbonyl bond ##STR4## formed with the carbon atom at the 7-position.