In the present study, we compared the performance of normal subjects in three age groups and of medicated Parkinson's disease patients on auditory selective attention processes. Two tone sequences were dichotically presented. Subjects responded to deviant tones in the attended location. Event-related potentials (ERPs) were recorded from nine scalp electrodes. The old group showed significant decline in hit rate, increase in N1 amplitude, and reduction in P3a, P3b, target negativity, and mismatch negativity amplitude. The amplitude and duration of late Nd increased with age, reflecting modulation of both attended and unattended standard ERPs. Although the middle group showed a similar pattern of ERP changes, the effects were generally nonsignificant. The Parkinson's disease group showed little further disruption of behavioral or ERP measures. However, Parkinson's disease affected late Nd in the direction opposite of that of aging, reflecting differential modulation of unattended standard positivity.