The effects of repeated, spaced, electroconvulsive stimulation (ECS) on rodent hippocampal synaptic plasticity was investigated in vivo. Long-term potentiation (LTP) was induced in intact rats using high-frequency perforant path stimulation, and measured by recording extracellular excitatory field potentials and population spikes evoked in the dentate gyrus by low-frequency stimulation before and after LTP induction. LTP induction appeared to be inhibited in animals which had received ECS. However, inspection and analysis of absolute excitatory postsynaptic potential and population spike size before LTP induction in ECS treated animals suggested that LTP may have already been induced as a consequence of seizure activity, reducing the degree to which further potentiation could be elicited experimentally.