Brains from 5 mice subjected to focal ischemia (2 hours)--reperfusion (22 h), revealed a significant increase (P < 0.01) in cells exhibiting DNA fragmentation (100-200 per section)--ipsilateral hemisphere compared to 0-3 per section found in the contralateral hemisphere and normal (n = 5) and sham operated (n = 5) mice. Neurons were the predominant cells (90-95%) exhibiting DNA fragmentation, and were primarily located in the inner boundary zone to the infarct. Apoptosis may contribute to the development of infarct after transient focal cerebral ischemia.