Medial temporal brain structures and hippocampus are associated with memory encoding and activate during registration and transfer of information within cortico-hippocampal circuits. Hippocampal dysfunction may cause learning and memory deficits as well as epileptogenesis. We monitored neuronal population activity of the human hippocampal formation with a whole-head magnetoencephalographic array. Attention-dependent hippocampal responses were elicited by auditory oddball stimuli. The most prominent responses emerged 200-500 ms after attended oddballs in good agreement with earlier intracranial recordings. Some activation of hippocampal source areas was seen at 60-100 ms. This completely non-invasive method permits access to cortico-hippocampal neural networks on the millisecond time scale during normal and abnormal brain activation and thus provides a new tool for evaluation of cognitive processes and pathological conditions involving hippocampal formation.