Hippocampal neurons, when pretreated with sublethal ischaemia, acquired tolerance to future treatments of normally lethal ischaemia. The level of transcription of the stress protein hsp70 was studied by in situ hybridization in control, ischaemic and tolerance-induced-ischaemic hippocampal neurons. Mongolian gerbils were subjected to single forebrain ischaemia for 2 min (2-min ischaemia group: sublethal) or 5 min (5-min ischaemia group: lethal). Other animals, were exposed to sublethal ischaemia for 2 min and were subjected subsequently to ischaemia for 5 min, 2 days later. Animals were sacrificed at 6, 12, 24 and 48 h following ischaemia and in situ hybridization was performed with thin cross sections including the hippocampus. Hybridization of the hsp70 probe in the control group (no ischaemia) was barely visible in the brain. In the 2-min ischaemia group, intense hybridization was seen in the CA1 sector from 6 through 24 h of recirculation but hybridization almost disappeared at 48 h of recirculation. In the 5-min and the double ischaemic groups, the intense induction in the CA1 sector occurred during the period from 6 through 48 h of recirculation. Our present study by in situ hybridization reveals high levels of transcription of hsp70 message following 5-min of ischaemia, however no protein was detectable based on our previous experiments employing immunocytochemistry. In tolerance-induced hippocampal neurons, both hsp70 message and protein are present. These results suggest that tolerance-induced neurons are modified such that the hsp70 gene is both transcribed and translated. These changes most likely give rise to the tolerance observed in the double ischaemia group.