Mature sperm cells have the spontaneous capacity to take up exogenous DNA. Such DNA specifically interacts with the subacrosomal segment of the sperm head corresponding to the nuclear area. Part of the sperm-bound foreign DNA is further internalized into nuclei. Using end-labelled plasmid DNA we have found that 15-22% of the total sperm bound DNA is associated with nuclei as determined on isolated nuclei. On the basis of autoradiographic analysis, nuclear permeability to exogenous DNA seems to be a wide phenomenon involving the majority of the sperm nuclei. In fact, the foreign DNA, incubated with sperm cells for different lengths of time, is found in 45% (10 min) to 65% (2 hr) of the sperm nuclei. Ultrastructural autoradiography on thin sections of mammalian spermatozoa, preincubated with end-labelled plasmid DNA, shows that the exogenous DNA is internalized into the nucleus. This conclusion is further supported by ultrastructural autoradiographic analysis on thin sections of nuclei isolated from spermatozoa preincubated with end-labelled DNA.