Human memory involves an interaction between transient working memory and a long-term store. We found electrophysiological evidence supporting the existence of two distinct mechanisms subserving these processes. Stimuli held in memory for less than 4 s generate large, early latency P300 potentials which may index activation of a frontally-mediated rapid working memory system. Stimuli held in memory for over 4 s selectively generate N400 potentials which may reflect activation of mesial temporal cortices involved in access to the long-term store. These results suggest that memory processing in the initial 10 s after stimulus detection involves at least two distinct distributed cortical-limbic systems.