The effects of inter-stimulus intervals on P300 from an oddball task (target and standard stimuli) and a single-stimulus task (targets only) employing simple visual stimuli were assessed in order to determine how a relatively long ISI affects event-related brain potentials (ERPs). Young adult subjects (n=16) responded by pressing a button to a visual target stimulus of each task condition. ISI was either 2.5 or 30 s and paradigm type was either the oddball or single-stimulus task. ERPs were recorded from the midline electrodes, with amplitude, mean area, and latency of the P300 and other components assessed. The results showed that P300 morphology was dramatically affected by task and ISI such that under the 2.5 s condition, the oddball paradigm produced typical ERP components, whereas the single-stimulus condition demonstrated minimal P300 amplitude. When ISI was 30 s, both the oddball and single-stimulus tasks produced robust P300 components but also evinced strong slow wave (SW) potentials, which contributed to the ERP measurement outcomes. It is concluded that P300 from visual stimuli can be elicited with both oddball and single-stimulus tasks when ISI is relatively long. ERPs from both paradigms produced appreciable SW activity, which needs to be considered when long ISI procedures are employed.