This study used magnetoencephalography to examine the possibility that different generators account for the long-latency event-related potential (P300), evoked by rare target and by rare non-target, novel visual stimuli, in a visual oddball counting task performed by seven subjects. As expected, P300 peak latency was longer in response to rare targets compared to novel, non-target stimuli. Two main source regions were found for the Target- as well as for the Novel-P300, one in the temporal and one in the occipital lobe. Centers of neural activity were observed in the vicinity of the superior temporal sulcus, in the hippocampal formation and parahippocampal gyrus and in the occipital extrastriate cortex. It appears that the brain structures which contributed to the generation of the P300 response to both the target and the novel visual stimuli overlapped to a great extent.