Computer modelling with an energy minimization procedure is used here to obtain stereochemical and energetic details for complexes of the dye Hoechst 33258 with different oligonucleotide sequences. An optimised model of the dye with d(A)5-d(T)5 is in conformity with previous proposed models. It has bifurcated hydrogen bonds between N2H and N4H of benzimidazole rings with N a of adenine and 0 2 of thymine. Relative binding energies with different oligonucleotides show preference for AT containing sequences, with an intermediate affinity between that for netropsin and distamycin-2. Reduced binding is observed at high ionic concentration. The benzimidazole rings are twisted with respect to the phenol ring in the optimal model. This gives desired curvature to the molecule which is stabilised by intermolecular forces.