The P3(00) event-related brain potential (ERP) was elicited with auditory and visual stimuli using an oddball task in which the subject indicated with a button-press response the occurrence of a target stimulus that occurred randomly on 20% of the trials and refrained from responding to a standard stimulus. A total of four trial blocks were collected for each stimulus modality condition. P3 amplitude from the target stimuli decreased reliably across trial blocks for both the auditory and visual stimulus condition, with no interaction obtained between stimulus modality and trial block. P3 latency was shorter for the auditory compared to visual stimulus conditions, but did not vary with trial block. No changes in amplitude or latency independent of the P3 effects were obtained for the other ERP components with trial block, although the usual modality differences were observed. The results suggest that P3 components elicited by auditory and visual stimuli both habituate for actively discriminated target stimuli. The theoretical implications are discussed in the context of previous findings.