Repetitive transcranial magnetic stimulation (rTMS) is a new method to investigate the relationship between cortical activity and cognitive processes. Contradictory findings exist concerning the inhibitory function of low frequency rTMS. In order to clarify this point, this study examines the impact of different duration of low frequency rTMS on event-related potentials (ERPs). In 17 subjects, auditory ERPs were measured before and after 1 Hz rTMS delivered over the left prefrontal cortex during 10 min (600 pulses) and 15 min (900 pulses), in comparison to a sham stimulation. Results showed that 15 min of 1 Hz rTMS induced a significant increase of P300 latency. No effect was found after both 10 min and sham application. There was no effect for early ERP components (N100, P200 and N200). This study confirms and extends that 1Hz rTMS produces a real inhibitory effect of the central nervous system only when the duration of the stimulation is about 15 min. Moreover, the data suggest that rTMS applied to the left prefrontal cortex modifies the speed of cognitive processing rather than the energetical aspect of information processing, and that cortical inhibition induced by the magnetic stimulation affects principally the controlled cognitive processes and not the automatic ones.