Large, ethidium bromide-loaded liposomes electrically pulsed in the presence of externally added DNA display the bright fluorescence of DNA-ethidium bromide complexes. Sonication of these Liposomes increases the fluorescence of trapped DNA-ethidium bromide complexes by no more than about 40%. These results are thus in agreement with a mechanism involving electropores for DNA uptake but do not support an alternative mechanism, invoking invagination and pinching-off of the lipid bilayer, through which internalized DNA is shielded from the liposome contents.