We aimed to see the consequences of habituation and posthabituation by means of repeatedly measuring the effects over a longer period. The study was performed on 27 healthy volunteers. The event-related potentials were recorded from the Fz, the Cz, and the Pz electrode sites. The rare tone-frequent tone probability ratio was 20%. All subjects were asked to press a button when they heard a rare tone. The test was continued until 20 artifact-free rare tones were averaged, which was accepted as 1 trial block. After 10 trial blocks were obtained in a sequential manner, the test was completed. This study has indicated that P300 amplitudes decrease with repeated stimulations, that is, there is a habituation period. As the test continues, the speed of amplitude decrease slows down and after a while it even starts to increase: that is, a dishabituation occurs. It is very likely that this habituation relates to a period of learning and dishabituation relates to a period of mental fatigue. In these processes, changes of amplitude and latency values reflect changes in amount of neuronal activation.