Long-term potentiation (LTP) is a form of synaptic plasticity that serves as a model for certain types of learning and memory. The role of the calcium-activated thiol proteases or calpains in the biochemical mechanism of LTP has been explored. We show that the extracellular application of two newly developed, highly potent calpain inhibitors, N-acetyl-Leu-Leu-norleucinal and N-acetyl-Leu-Leu-methioninal, block LTP in both the Schaffer collateral-CA1 synaptic zone of the rat hippocampal slice and in perforant path-stimulated dentate granule cells in the intact hippocampus. The inhibitors do not affect baseline synaptic transmission and block LTP in the slice preparation if applied before but not after tetanic stimulation. The calpain inhibitor leupeptin is less potent than the above peptides but also blocks LTP if applied at a sufficient concentration.