In the hippocampal formation GABAergic inhibitory interneurons have a major role in the synchronization of neuronal activity and are involved in the generation of large-scale network oscillations. Thus, interneurons function as a 'clock' that dictates when principal cells fire during suprathreshold excitatory drive. Interneurons receive strong excitatory innervation from glutamatergic neurons and it has been much debated whether these synapses show mechanisms of long-term plasticity similar to those found at principal-cell synapses. Recent findings support the lack of conventional forms of LTP and LTD in most interneurons, partly owing to the distinct anatomical and neurochemical features of interneuronal excitatory synapses. The uncommon properties of excitatory synapses on interneurons might be required for their functioning as accurate and reliable neuronal oscillators.