Working memory is responsible for the short-term storage and online manipulation of information necessary for higher cognitive functions, such as language, planning and problem-solving(1,2). Traditionally, working memory has been divided into two types of processes : executive control (governing the encoding manipulation and retrieval of information in working memory) and active maintenance (keeping information available ’online’). It has also been proposed that these two types of processes may be subserved by distinct cortical structures, with the prefrontal cortex housing the executive control processes, and more posterior regions housing the content-specific buffers (for example verbal versus visuospatial) responsible for active maintenance(3,4). However, studies in non-human primates suggest that dorsolateral regions of the prefrontal cortex may alsb be involved in active maintenance(5-8). We have used functional magnetic resonance imaging to examine brain activation in human subjects during performance of a working memory task. We used the temporal resolution of this technique to examine the dynamics of regional activation, and to show that prefrontal cortex along with parietal cortex appears to play a role in active maintenance.