An antibody against rat calbindin-D,,,, a calciumbinding protein present at high concentration in certain neurons of the central and peripheral nervous systems, was used to determine the progression of the pathological events in the rat hippocampus following experimental cerebral ischemia. Calbindin-D,,, immunoreactivity is present in dentate granule cells and in the CA1-CA2 pyramidal cells. CA1 subfield contains a higher proportion of calbindin-D,,,-positive pyramidal cells than does the CA2 subfield and CA1 cells are more immunoreactive than the CA2 cells. The pyramidal cells of the CA1 and CA2 subfields are vulnerable to ischemia. The cells in the CA1 became necrotic within 3-4 days after ischemia while those of the CA2 became necrotic within 2 days. There was a concomitant decrease in calbindin-D,,, immunoreactivity in the whole hippocampal regio superior after ischemia which peaked 3 days postischemia. The difference in CA2 and CA1 vulnerability seemed to be inversely correlated with the calbindin-D,,, contents of the CA2 and CA1 pyramidal cells. The decrease in the calbindin-D,,, contents of these neurons was accompanied by cell damage. We therefore suggest that calbindin-D,,, is an important factor for the survival of pyramidal cells in the hippocampal formation after ischemia.