Within session and between session reliabilities of the task-related component (P3b) of the P3 measures (amplitude, area and latency) and their habituation across eight sessions separated by 7-10 days, except for an interval of 1 month between the 6th and 7th sessions, were studied based on the difference waves, which were obtained by subtracting the ignored infrequent event-related potentials (ERP) from the target ERP elicited by a standard auditory oddball paradigm with eyes-open or eyes-closed conditions in 10 normal subjects. The within session reliabilities represented as Pearson correlation coefficients (r) were 0.57-0.66 for the three measures except for those for the latency and amplitude under the eyes-closed condition. The between session reliabilities expressed as intraclass correlation coefficients (R) ranged from 0.54 to 0.60 except for that for latency under eyes-closed conditions. Long-term habituation occurred within the first six sessions for the P3b amplitude and area, and dishabituation took place in the 7th session after an interval of 1 month, whereas no such phenomenon was observed for the P3b latency. Implications of the present results are discussed in terms of the clinical application of the P3 measures.