A direct correlation between the duration of arterial hypotension (40 mmHg) and the activity value of nuclear Ca2+/Mg(2+)-dependent DNA-endonucleases was revealed in dogs exposed to hemorrhagic shock. Thus, an increase in the activity of endonucleases was significant after 2 h of arterial hypotension. During that period there activity amounted to 4149 +/- 295 units/mg of protein instead of 2839 +/- 231 units/mg protein in the control (P < 0.01). With the prolonged hemorrhagic shock, the activity of DNA-endonucleases continued to increase. By 4 h of hemorrhagic shock it reached 5021 +/- 443 units/mg protein and by 6 h, 8661 +/- 338 units/mg protein which was three times the control values (P < 0.01). An electron-microscopic study of the structure of neuron nuclei karioplasm showed that by 2-4 h of hemorrhagic shock some neurons showed reduced chromatin content, and the nuclear matrix had clarified with the formation of electron-translucent empty zones. We think that the sharp increase in the nuclear endonuclease activity and the subsequent endonucleolysis can be regarded as a pathogenetic factor in the destructive processes taking place in the nuclei of cortex neurons during prolonged periods of hemorrhagic shock.