We investigated the acute effects of caffeine (500 mg) on event-related potentials (ERP) in 10 healthy subjects using standard oddball and single-tone paradigms. Event-related potentials were recorded before oral ingestion of caffeine or placebo and 30 min and 210 min after. The oddball paradigm, but not the single-tone paradigm, showed that the P300 amplitude and the area were significantly increased 30 min after ingestion of caffeine and significantly decreased 30 min after ingestion of placebo. The effects of caffeine disappeared at 210 min. Neither the P300 latency nor the reaction time changed significantly with the oddball paradigm. However, the reaction time was shortened 30 min after ingestion of caffeine with the single-tone paradigm. These findings suggest that the caffeine-induced increase in the P300 amplitude may have resulted from the increased allocation of attentional resources to the discriminating process which was not, however, accompanied by facilitation of the process and that caffeine may specifically stimulate the discriminating process involved in the oddball paradigm. In addition, the simple psychomotor performance of buttonpressing in response to a tone signal was accelerated by caffeine.