SUMMARY1. Long-term potentiation (l.t.p.) was studied in area CAl of rat hippocampal slices during development at post-natal days 1-8, 15 and 60. Tetanic stimulation at 100 Hz for 1 s was delivered to the fibres in stratum radiatum and the time course of potentiation was recorded in stratum pyramidale for 20 min after tetanus. L.t.p. was measured at 20 min post-tetanus as an increase in the amplitude of the population spike.2. The time course and magnitude ofpost-tetanic potentiation (p.t.p.) differed with age. For 60-day-old animals p.t.p. was seen as a maximally potentiated response immediately post-tetanus that declined to a smaller potentiated response by 5 min post-tetanus. For animals younger than 15 days the response was also maximally potentiated immediately post-tetanus with subsequent decline. However, the duration of maximal potentiation was shorter and the magnitude was less. A different time course ofp.t.p. was observed at 15 days. The maximal potentiation was approximately equal to that seen at 60 days, but instead of declining, the response remained maximally potentiated throughout the entire post-tetanus monitoring period.3. L.t.p. was first observed at post-natal day 5, and by post-natal days 7 and 8 substantial levels of l.t.p. were seen consistently. The greatest magnitude of l.t.p. was found at 15 days, and was considerably more than that produced at 60 days.4. When the duration of l.t.p. was monitored for longer than 20 min the response declined back to pretetanus levels by 1-11 h for animals younger than 15 days. In 15-day-old rats the response remained maximally potentiated for the full 72 min that it was monitored, with no decline.5. In control experiments of low-frequency stimulation (l.f.s.) at 1/15 s for 100 stimuli, hippocampal slices from 60-day-old animals showed response elevation. In contrast, l.f.s. resulted in response decrement over time for slices from 5-15-day-old animals.6. Three measures ofpretetanus excitability in area CAI suggested an increase with age. The stimulus intensity required for field excitatory post-synaptic potential (e.p.s.p.) threshold declined, the magnitude ofthe maximal population spike amplitude increased, and the population spike latency decreased. These results suggest that the magnitude of l.t.p. is not strictly related to the pretetanus excitability of CAl cells.

High-frequency, repetitive, auditory stimulation was used to determine whether induction of a long-lasting increase of the human auditory evoked potential (AEP) was possible. Recording non-invasively with electroencephalogram scalp electrodes, stable increases in amplitude were observed in the N1 component of the AEP, which is thought to reflect activity within auditory cortex (N1). The increase was maintained over an hour and was shown to be independent of alterations in the state of arousal. This is the first demonstration of the induction of long-lasting plastic changes in AEPs, and suggest that this represents the first direct demonstration of long-term potentiation in the auditory cortex of normal, intact humans.

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