The P3(00) event-related brain potential (ERP) was elicited with an auditory discrimination paradigm under two response task conditions in which the probability of the target tone was always 0.20. Subjects either counted the occurrence of each target tone or pressed a button to the target in different conditions. No response was made to the standard tone. Consecutive blocks of trials were obtained to assess habituation of P3 amplitude. The count task demonstrated less habituation than the button-press task, with a strong interaction obtained between response mode and trial block for all electrode sites. The findings suggest that habituation of P3 amplitude is sensitive to the amount of attentional resources allocated to the processing of a target stimulus, with more resources required for a count task compared to fewer needed for a button-press response.