Slices from rat hippocampus were incubated with the caspase‐3 inhibitor N‐benzyloxycarbonyl‐Asp‐Glu‐Val‐Asp fluoromethylketone (Z‐DEVD‐FMK) or with the inactive peptide N‐benzyloxycarbonyl‐Phe‐Ala fluoromethylketone (Z‐Phe‐Ala‐FMK) for 30 min. The peptides changed neither input–output curves nor paired‐pulse effects at 70‐msec interpulse intervals, nor amplitudes of pop spikes in the CA1 region 1.0–6.9 hr after the incubation. Slices taken 1.0–1.4 hr after Z‐DEVD‐FMK or inactive peptide treatment demonstrated similar long‐term potentiation (LTP) curves; however, LTP was suppressed significantly (P < 0.001) 1.5–3.4 hr after Z‐DEVD‐FMK treatment when compared to the corresponding inactive peptide group. LTP magnitude correlated with time after Z‐DEVD‐FMK (r = −0.74; P < 0.02) but did not depend on time after the inactive peptide treatment. After 3.5 hr, LTP was blocked completely. Z‐DEVD‐FMK did not have a significant effect on presynaptic function. The results are the first evidence that inhibition of caspase‐3 significantly decreases or fully blocks LTP in the CA1 region and suggest that caspase‐3 is essential for LTP. Candidate caspase‐3 substrates that may be cleaved for LTP induction and maintenance are discussed. © 2003 Wiley‐Liss, Inc.