The kinetics of sperm nuclear decondensation induced by the action of physiological concentrations of heparin and glutathione was studied by comparing two rodents: the rat, with very stable protamine P1 containing chromatin (class I nuclei), and the mouse, with protamine P1 and protamine P2 (class II nuclei). Sperm suspensions were incubated at different temperatures (37, 40, 43, and 46 degrees C) in media while keeping a constant concentration of either heparin or GSH and increasing concentrations of the other reagent. Spermatozoa nuclei without any treatment incubated for 72 h appear densely condensed. Swelling of mouse spermatozoa nuclei was observed after 30 min of incubation in the presence of efficient concentrations of heparin-GSH. The extent of this time lag was significantly reduced at higher temperatures. This behavior was also observable in the rat, but required time lags of 3-4 h. Electron microscopy observations showed that the pattern of nuclear decondensation was different in both animal species. Mice sperm nuclei initiates its decompaction by the peripheral regions and this behavior remains until late stages of decondensation. On the contrary, rat spermatozoa nuclei decondense initially at the central part of the nuclei while the periphery remains condensed, showing numerous residues of densely packed chromatin. In both cases, the chromatin is organized into "hub-like" nuclear bodies joined by a network of chromatin fibers.