The mechanism whereby cationic lipids destabilize cell membranes to facilitate the intracellular delivery of macromolecules such as plasmid DNA or antisense oligonucleotides is not well understood. Here, we show that cationic lipids can destabilize lipid bilayers by promoting the formation of nonbilayer lipid structures. In particular, we show that mixtures of cationic lipids and anionic phospholipids preferentially adopt the inverted hexagonal (H(II)) phase. Further, the presence of 'helper' lipids such as dioleoylphosphatidylethanolamine or cholesterol, lipids that enhance cationic lipid-mediated transfection of cells also facilitate the formation of the H(II)phase. It is suggested that the ability of cationic lipids to promote nonbilayer structures in combination with anionic phospholipids leads to disruption of the endosomal membrane following uptake of nucleic acid-cationic lipid complexes into cells, thus facilitating cytoplasmic release of the plasmid or oligonucleotide.