The goal of this study was to analyze the spatiotemporal expression of heat shock protein 20 (Hsp20) and its phosphorylation in gerbil brain after transient forebrain ischemia. Brain sections from Mongolian gerbil killed 24, 48, 72 and 96 hours and 2 weeks after ischemia (n=5 in each experimental group) were evaluated with immunohistochemical and in situ DNA end-labeling [terminal 2'-deoxyuridine 5'-triphosphate nick end-labeling (TUNEL)] techniques. Ischemia-associated Hsp20 expression was observed 24 and 48 hours later in the area of the stratum radiatum and then disappeared by 72 hours. This staining appeared along the lines of apical dendrites. Hsp20 staining in the stratum pyramidale was observed again 2 weeks after ischemia. Strong immunoreactivity for phosphorylation markers was observed in the stratum pyramidale 2 weeks after ischemia, whereas no staining was seen at either 24 or 48 hours after ischemia. Fragmented DNA was observed in nuclei and apical dendrites of CA1 pyramidal neurons by TUNEL method between 72 and 96 hours after reperfusion. The emerging expression of the Hsp20 protein within the restricted location of CA1 before the fragmented DNA transport suggests the strong relationship between Hsp20 and CA1 neuronal cell apoptosis. These findings imply a two-phase role of Hsp20 in brain ischemia: an acute phase before DNA fragmentation and a subacute phase 2 weeks after ischemia. The former may be associated with apoptosis with fragmented nuclear DNA transport into neuronal fibers and the latter associated with glial response to ischemic insult. Phosphorylation of Hsp20 might contribute to the subacute phase but not to an acute phase.