We have used the hippocampal slice preparation to study cell properties, synaptic organization and synchronized population bursts and oscillations. Our methods are those of electrophysiology and computer modelling. The results shed light on the mechanisms of certain epilepsy types and brain waves that occur in the whole animal. In addition, detailed information on cellular organization of the hippocampus is critical for the evaluation of computational brain theories. To illustrate this point, we shall discuss a particular computational theory of the CA3 region of the hippocampus. This theory concerns the way recurrent excitatory synaptic connections in the hippocampus might be used to define distance in a non-topographic map of the environment. If correct, the theory has specific physiological implications. © 1992 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.