Spike timing-dependent plasticity (STDP) of synaptic strength is thought to require the coincidence of presynaptic glutamate release and postsynaptic depolarisation through back propagating action potentials (Magee & Johnston, 1997; Markram et al. 1997). The exact timing of these presynaptic and postsynaptic spikes has been shown to dictate the direction and magnitude of the resulting plasticity at a variety of different synapses. However, evidence for this phenomenon in acute hippocampal slices is less clear. There is also evidence that modulatory inputs to the hippocampus, such as acetylcholine (ACh), can alter the behaviour of back propagating action potentials (Tsubokawa & Ross, 1997). Therefore we examined if ACh can modulate STDP in the hippocampus. Hippocampal slices (300-400μm) were prepared from 2 week old male Wistar rats and whole cell patch clamp recordings made from visualised CA1 pyramidal neurones at room temperature (23°C) in the presence of 50μM picrotoxin. In voltage clamp, baseline EPSCs from 2 independent pathways were elicited by stimulation in stratum radiatum at 0.1Hz. In current clamp, synaptic plasticity was induced by 4 trains of 100 stimuli given at 1 min intervals to the proximal pathway. Each stimulus consisted of a presynaptic input coupled with a postsynaptic action potential induced by a 2ms 0.5A current injection. The spike timing interval (STI) between the presynaptic and postsynaptic event in each stimulus was measured as the difference between the onset of the EPSP and the peak of the action potential. The EPSC amplitude in both pathways was then monitored for a further 30min. Pairs of presynaptic and postsynaptic spikes with a STI of +6 to +13ms were able to induce homosynaptic LTP when the stimuli were delivered at 20Hz (140±17%, n=11, p<0.05) or 10Hz, (146±20%, n=4, p<0.05) but not at 5Hz (113±18%, n=8, p=0.5) or 1Hz (95±21%, n=4, p=0.8) thus demonstrating the frequency dependence of STDP. Changing the timing of the stimuli given at 10Hz also altered synaptic plasticity. STIs of +6.2ms to +13ms resulted in potentiation whereas STIs of +2.5ms to +6.2ms resulted in a small non-significant depression (76±45%, n=6, p=0.3). When stimuli with STI values of +8-13ms were delivered at 10Hz in the presence of 1μM carbachol, the ACh receptor agonist, the synaptic potentiation seen in the absence of carbachol was blocked (96±19%, n=8, p=0.8). All statistical values shown are paired Student’s t tests. These results demonstrate that STDP in acute hippocampal slices is frequency and timing dependent requiring stimuli repeated at 10Hz or more with STIs between +6.2 and +13ms. We also show that STDP is inhibited by activation of ACh receptors. This is contrary to reports that activation of ACh receptors generally increases the excitability of CA1 pyramidal neurons and increases the back propagation of action potentials.