Coherent oscillations, in which ensembles of neurons fire in a repeated and synchronous manner, are thought to be important in higher brain functions. In the hippocampus, these discharges are categorized according to their frequency as theta (4-10Hz), gamma (20-80 Hz) and high-frequency (approximately 200 Hz) discharges, and they occur in relation to different behavioural states. The synaptic bases of theta and gamma rhythms have been extensively studied but the cellular bases for high-frequency oscillations are not understood. Here we report that high-frequency network oscillations are present in rat brain slices in vitro, occurring as a brief series of repetitive population spikes at 150-200 Hz in all hippocampal principal cell layers. Moreover, this synchronous activity is not mediated through the more commonly studied modes of chemical synaptic transmission, but is in fact a result of direct electrotonic coupling of neurons, most likely through gap-junctional connections. Thus high-frequency oscillations synchronize the activity of electrically coupled subsets of principal neurons within the well-documented synaptic network of the hippocampus.