Stopped-flow fluorescence studies were performed to investigate the kinetics of formation of complexes between DNA and cationic vesicles. We followed the kinetics as a function of charge ratio and lipid vesicle composition. Binary mixtures of a saturated cationic lipid, dioctadecyldimethlammonium bromide (DODAB), and a zwitterionic lipid, dioleoylphosphatidylethanolamine (DOPE), were used to prepare small unilamellar vesicles at 25 degreesC. The binding followed first-order reactions and occurred stepwise. The results presented indicate the existence of a three-step mechanism for cationic vesicle "binding" to DNA: first DNA rapidly adsorbs onto the oppositely charged membrane surface (milliseconds), and thereafter, on a longer time scale (seconds), the formed DNA-lipid complex continues to aggregate and grow; a third step is also observed, which seems to be due to rearrangement of the complex with a further change of the DNA conformation. The time constant of each step was determined by the relative amounts of DNA and cationic vesicles and the composition of the lipid bilayer of the vesicles.