The effects of age, stimulus modality and event probability on event-related potentials (ERP) were studied in 12 young and 12 elderly healthy subjects. The ERP were recorded from 15 electrodes referred to linked ears. Results showed that both amplitude and latency of the P300 component are affected by aging. Study of the latency of the earlier ERP components in the two age groups revealed that the P300 delay was not imputable to a delay of the earlier components. P300 amplitude and latency were also affected by event probability and stimulus modality: infrequent stimulus involved higher and later P300, but this effect was more pronounced in the young than in the old group; higher and later P300 were also recorded during the visual task compared to the auditory. Topographical repartition of the brain wave revealed a predominance of the central sites (Fz, Cz, Pz). The findings are discussed in relation to the sensitivity of the ERP assessment procedures in age related modifications of information processing.