The auditory event-related potentials (ERP's) of young, middle-aged and elderly subjects were measured over Fz, Pz, C3 and C4 in two different rare tone conditions. In the fixed condition, the rare tone occurred predictably, every fourth stimulus. In the random condition, the rare tone was presented unpredictably, with 1:4 probability. Large amplitude late positive waves (P3's) of middle aged subjects (N = 22) were present in the random condition at all placements, but absent in the fixed condition. Elderly subjects (N = 23) responded identically to both rare tone conditions at all placements. Young subjects (N = 7) had large amplitude P3 responses to both random and fixed conditions at all placements except Pz. Over Pz, young subjects had patterns similar to middle-aged subjects, with large P3's to the random rare tone but not the fixed rare tone. Elderly subjects may not differentiate the two conditions, either because they have less efficient memory, or because they primarily attend to the global probability of rare tone occurrence. The results with young subjects suggest that recent memory processes involved in discriminating rare tone conditions initially develop over the posterior (Pz) areas.