We examined the effect of semantic task repetition and of alternating between tasks on cerebral blood flow in three H215O positron emission tomography experiments. We found that repeatedly performing semantic tasks resulted in a reduction in cerebral blood flow to the left temporal association cortex similar to that found in priming experiments even though here there was no repetition of stimuli. Although the same effect was found in two different tasks (word meaning judgments and picture naming), it was only present when the same task was repeated on consecutive scans and not when the subjects alternated from scan to scan between tasks. We propose that there is a neural efficiency which develops in the association cortex of the temporal lobe as a result of repeatedly performing a semantic task. This efficiency is abolished by interruptions such as performing a different task. Hum Brain Mapp, 2008. © 2007 Wiley-Liss, Inc.