Lehmann has demonstrated that EEG topography can be used to segment EEG map series into a sequence of spatially stationary segments characterized by location of potential maxima and minima. We employed topographic segmentation techniques to study 9 channel EEGs recorded from 11 medication-free schizophrenic patients and 10 normal controls during resting and active task conditions, retesting 8 patients after neuroleptic treatment. To define EEG segments, average reference potential maps corresponding to global field power peaks in theta, alpha, and low beta activity were classified according to locations of extreme minimum and maximum values. Normals and schizophrenics did not differ in the number or types of switches between segments, or the frequency of hemisphere crossing of potential extrema. However, EEGs of normal subjects were characterized by significantly more (P less than 0.003) unused theta segment types (of a theoretically possible 36). Moreover, medication significantly (P less than 0.02) increased the number of unused theta segment types in EEGs of schizophrenics. We interpret these findings as evidence of increased spatial variability of brain electrical activity in schizophrenics and discuss their functional implications.