Brief bilateral carotid occlusion in the gerbil produces forebrain ischemia that results in almost complete neuronal destruction in the CA1 sector of the hippocampus. Treatment with phenytoin (200 mg/kg) blocked the ischemia-induced neuronal death. The average density of CA1 pyramidal neurons (cells/mm CA1) was 253.6 +/- 4.4 in the sham surgery group, 12.3 +/- 3.4 in the ischemia group, and 119.5 +/- 16.6 in the group treated with phenytoin before ischemia. Thus, phenytoin reduced ischemia-produced neuronal loss in hippocampal CA1 by 44.4% (P less than 0.001). The plasma levels of phenytoin that produced this effect ranged from 28.1 to 45.0 mg per liter, with a mean phenytoin level of 34.7 +/- 1.7 mg/l (n = 10). The results suggest that phenytoin may be a clinically useful cerebroprotective agent.