We used an event-related brain potential (ERP) technique developed by Hillyard et al. (1973) to test abilities to attenuate irrelevant stimuli and to detect target stimuli. Subjects, 12 healthy old (80.3 years) and 12 healthy young adults (22.0 years), heard 1500 Hz tones in one ear and 800 Hz tones in the other ear. Infrequently, the pitch of either tone was raised. During one run, infrequent tones in the right ear were targets, and in the other run those in the left ear were targets. Subjects counted targets. For both groups, an early component of the ERP (N1) was larger to tones in the attended ear than in the unattended ear, and a later component (P3) was largest to the target. This suggests that both groups can attenuate irrelevant stimuli and can use stimulus probability information in this task. That P3 was later for old subjects suggests that they take longer to decide stimulus relevance.