The structural changes that occur in chromatin DNA after ischemic brain injury are poorly understood. The presence of oligonucleosome fragments that are recognized as the characteristic DNA ladder has been demonstrated in global and focal ischemia, associated or not with random DNA fragmentation. Using pulsed-field gel electrophoresis, which improves DNA separation, we have now detected initial stages of DNA fragmentation that occur already 6 h after reversible focal cerebral ischemia in rats. This result confirms that internucleosomal DNA fragmentation precedes random DNA fragmentation in vulnerable striatal and cortical neurons following reversible focal cerebral ischemia.