The calpains are calcium-dependent intracellular proteases that are activated in a number of pathogenic conditions. We tested the capacities of protease inhibitors, calpain inhibitor I and leupeptin, to protect against the neuronal degeneration caused by cytotoxic hypoxia or transient global cerebral ischemia. Primary neuronal cultures were prepared from embryonic chick telencephalon, and cytotoxic hypoxia was induced by adding 1 mM NaCN to the culture medium for 30 min. Global ischemia was induced in rats by clamping both carotid arteries and lowering the arterial blood pressure to 40 mmHg for 10 min. Both calpain inhibitor I and leupeptin protected neurons against ischemic and hypoxic damage. Neuroprotection was indicated by increased cell viability and protein content in the cultures, and fewer damaged neurons in the hippocampal CA1-subfield. Thus, blockade of proteolysis can protect neurons against cytotoxic and ischemic damage.