The effects of two benzodiazepines, diazepam and triazolam, on long-term potentiation were tested in slices of hippocampus and piriform cortex. The drugs had little influence on baseline synaptic responses but both were very effective in blocking LTP elicited by theta pattern stimulation. The effects were fully reversible upon washout. Diazepam reduced the increase in burst responses that occurs during theta stimulation and thus appears to interfere with the initial triggering events for long-term potentiation. This may reflect the enhancing action of the drug on GABA-mediated inhibitory potentials. Triazolam did not detectably change the burst responses elicited by theta pattern stimulation. Experiments with slices of piriform cortex indicated that triazolam also failed to disrupt the development of long-term potentiation but instead caused the potentiation to decay back to baseline in 15-30 min. Triazolam thus seems to act on the mechanisms that stabilize long-term potentiation. These results provide a possible explanation for the amnestic effects of benzodiazepines in humans and animals and support the hypothesis that long-term potentiation contributes to memory encoding.