Linear transistor arrays on a silicon chip were used to map evoked extracellular field potentials in organotypic brain slices of rat hippocampus. The shape and amplitude of the transients were similar to those from records with micropipette electrodes. The spatial resolution was 21 and 4.6 microm. The sampled profile of the field potential showed a wide and shallow trough of transients in the stratum radiatum and a narrow but higher ridge in the stratum pyramidale. Due to the high resolution, the profile could be interpreted quantitatively in terms of synaptic currents. Transistor chips may become a novel tool for neurophysiological and pharmacological studies in brain slices.