The interaction of Ethidium Bromide (EtBr) with double-stranded (ds-) and single-stranded (ss-) poly[d(A-T)] was studied in different ionic strengths solutions. Optical spectroscopy and Scatchard analysis results indicate that the ligand interacts to both helix and coiled structures of the polynucleotide by ?strong? and ?weak? binding modes. The association parameters (binding constant ? K ? and the number of nucleotides corresponding to a binding site ? n) of the strong type of interaction were found to be independent of Na⁺ concentration. Weak interaction occurs at low ionic strength and/or high EtBr concentration. Estimated binding parameters of EtBr with ss- and ds-polynucleotide are in good agreement with those for EtBr-B-DNA complexes. Data obtained provided an evidence for a stacking interaction of EtBr with single stranded poly[d(A-T)].

Key words: Ethidium bromide, Double-stranded (ds-) and single-stranded (ss-) poly[d(A-T)], Stacking interaction, Strong and weak binding.