Cationic polymers bind DNA and form compacted nanoparticulates (i.e., polyplexes). Polyplexes augment DNA delivery into the cells as a nonviral method of gene therapy. DNA packing and release are the key factors in polyplex-mediated gene delivery, but they are poorly understood due to the lack of physical methods of investigation. We used time-resolved fluorescence spectroscopy to study poly(ethylenimine) (PEI) and poly(L-lysine) (PLL) polyplexes. Analysis of fluorescence lifetimes and time-resolved spectra revealed that DNA exists in several different states in PEI polyplexes and only in one tightly bound state in PLL polyplexes. The observed difference in the nature of the polyplexes may explain why PEI releases DNA more easily than PLL even though both polycations condense DNA effectively. The present method utilizing time-resolved fluorescence spectroscopy gives information on the specific interactions between DNA and the cationic polymers in the polyplexes. This kind of information is very important in the development of biologically effective nonviral systems for DNA delivery.