The effect of synthetic cationic lipids on the structure of DNA was studied. The fluorescence enhancement of ethidium bromide on intercalation into DNA was suppressed by the addition of bilayer-forming lipids, but not by micellar ones. Results on the fluorescence depolarization index suggest that ethidium bromide is not released from DNA by lipids intercalated into DNA. CD spectra of the DNA-lipid complexes revealed that the structure of DNA was changed only by bilayer-forming lipids at temperatures lower than their Tc values. Thus, the conformation of DNA is forced to change by cationic lipids forming the rigid bilayer membrane so that ethidium bromide fluorescence might be reduced, and the conformation can be controlled by selection of the appropriate lipid and temperature.