The function of neuronal networks is determined by system properties like coupling of neuronal aggregates by synchronization of spontaneous discharges. In order to study such network properties, recordings of the spatial properties of neuronal networks are necessary. We investigated the possibility of analyzing the functional properties of brain slices by optical methods and microelectrodes. Stimulation-induced intrinsic optical signals (IOSs) were monitored using the dark-field configuration of a conventional light microscope. For electrical recordings, glass microelectrodes were placed within the slice. Ion-selective microelectrodes were used to measure activity-induced changes in extracellular potassium concentration. The experiments revealed that electrical stimulation of the slice induces alterations of IOSs. The IOSs were accompanied by extracellular field potential deflections which are composed of slow and fast components. At corresponding locations transient elevations of the extracellular potassium concentration occurred. Recent experiments revealed that the IOS monitors changes of extracellular space volume. This property of IOSs could further be used to demonstrate that the brain slice possesses a functionally coupled network of glial cells which serve to transport potassium ions from areas of excitation into neighbouring brain regions.