Periodic alterations of event-related potentials (ERPs) were studied during 'oddball' tasks. Sequences of randomly intermixed frequent (non-target) and rare (target) stimuli were presented. In visual experiments, these were flashes of light of two different colors. In auditory tests there were two tones of different frequencies. The instruction was to keep a mental count of each target stimulus. To study the alterations of the 'state of the brain' produced by target detection, responses to non-targets immediately following targets were compared with responses to an eighth subsequent non-target stimulus. To evaluate the effect of such 'brain states' on responses to stimuli of a different modality, additional visual stimuli (probes) were delivered after both auditory and visual 'oddball' stimuli. It was found that responses to the eighth presentation of non-target stimulus were preceded by significant negative shift of recorded potential. This shift was smaller before the responses to non-targets immediately following the presentation of target stimuli. The difference was significant both in auditory and visual tests. Responses to 'oddball' stimuli were little affected: only the reduction of P200 peaks in 'after target' responses was significant in visual tests. Responses to probes showed stronger effects: when visual probes followed visual 'oddball' stimuli, all three components measured (N100, P130 and P200) were shifted positively in responses to eighth presentations of non-targets. When visual probes were presented in auditory tests, only the amplitude of the N100 component was significantly affected.