Pyrene-labeled 2'-O-methyloligoribonucleotide (OMUpy) and 5'-Ru(II) complex labeled oligodeoxyribonucleotie (Ru-probe) were prepared as the fluorescence probes to detect acceptor sites of antisense molecules on native folded RNA under the physiological condition. OMUpy showed the remarkable increase of the fluorescence intensity (334-fold at 375 nm) only when hybridized with complementary oligoRNA. When OMUpy was applied to E. coli 16S-rRNA, the fluorescence intensities were increased in a sequence specific manner. The rotational correlation time of Ru-probe complementary to 16S-rRNA were largely dependent on the sequence of Ru-probe. These results suggest that the accessibility of the antisense molecules for RNA can be evaluated by those fluorescent probes.