Although Cornu Ammonis (CA) 1 neurons of the hippocampus are known to be vulnerable to transient ischaemia, the mechanism of ischaemic neuronal death is still unknown, and there are very few strategies to prevent neuronal death at present. In a previous report we demonstrated μ-calpain activation at the disrupted lysosomal membrane of postischaemic CA1 neurons in the monkey undergoing a complete 20 min whole brain ischaemia. Using the same experimental paradigm, we observed that the enzyme activity of the lysosomal protease cathepsin B increased throughout the hippocampus on days 3–5 after the transient ischaemia. Furthermore, by immunocytochemistry cathepsin B showed presence of extralysosomal immunoreactivity with specific localization to the cytoplasm of CA1 neurons and the neuropil of the vulnerable CA1 sector. When a specific inhibitor of cathepsin B, the epoxysuccinyl peptide CA-074 (C18H29N3O6) was intravenously administered immediately after the ischaemic insult, ≈ 67% of CA1 neurons were saved from delayed neuronal death on day 5 in eight monkeys undergoing 20 min brain ischaemia: the extent of inhibition was excellent in three of eight and good in five of eight monkeys. The surviving neurons rescued by blockade of lysosomal activity, showed mild central chromatolysis and were associated with the decreased immunoreactivity for cathepsin B. These observations indicate that calpain-induced cathepsin B release is crucial for the development of the ischaemic neuronal death, and that a specific inhibitor of cathepsin B is of potential therapeutic utility in ischaemic injuries to the human CNS.