The psychophysiological basis of problem size effect in the arithmetical processing of additions and subtractions was studied with event-related brain potentials (ERP). Subjects were presented sequences of seven numbers, and ERPs elicited to the sixth number were analyzed. Two variables were manipulated: operation type (addition and subtraction) and problem size (by adding or subtracting 2, 3 or 4). Results showed two phases in the ERP pattern: an early phase, appearing to reflect automatic processing involved in stimulus identification, and a positive slow wave, believed to be a computing indicator of the subsequent calculation. The amplitude of this positive slow wave was modulated by the problem size (the more problem size, the larger the amplitude), suggesting that the amplitude of this slow wave indexes the activation of the cerebral network underlying problem size effect.