Single-photon emission computerised tomography (SPECT) was used to investigate whether pre-frontal cerebral blood flow in normal adults is increased during planning activity. A subtraction technique was used in which regional cerebral blood flow (rCBF) was measured in subjects during a computerised version of the Tower of London task. Both rCBF and performance on this task were compared to a motor control condition requiring the same responses and using the same visual display. The level of rCBF was significantly increased in the left pre-frontal cortex during the Tower of London task. In addition, subjects who took more time planning their moves, and less moves to complete a problem had a significantly higher level of rCBF in the left pre-frontal cortex. Subsequent execution latencies for the task were correlated negatively with both left and right rCBF.