1. Potential changes evoked by stimulation of the perforant path have been recorded in the dentate area of the hippocampal formation in chronically prepared unanaesthetized rabbits.2. Components attributed to excitatory synaptic current flow and to action potentials in the granule cell population were distinguishable, with characteristics largely the same as in anaesthetized rabbits.3. Stimulation at 15/sec for several seconds usually led to the granule cells being more effectively activated by the individual stimuli of the train (;frequency potentiation'). Single stimuli then commonly produced multiple discharges in the granule cell population.4. After single periods of stimulation at 15/sec for 15-20 sec there was on 26% of the occasions (41% of those on which there was good frequency potentiation) a long-lasting potentiation of the responses to subsequent stimuli, lasting from 1 hr to 3 days.5. After a further 20% of the periods of repetitive stimulation there was a shorter lasting potentiation, and after 8% there was a short lasting depression.6. The potentiation, when present, was characterized by some or all of the following changes: increases in the amplitudes of the synaptic wave and population spike, reduction in the latency of the population spike, and reductions in the variability of the characteristics of the population spike.7. During the long-lasting potentiation there was an increase in the excitability of the post-synaptic cells and, on some but not all occasions, an increase in the extracellular current flow produced directly by synaptic action.