To investigate the role of temporal processing in language lateralization, we monitored asymmetry of cerebral activation in human volunteers using positron emission tomography (PET). Subjects were scanned during passive auditory stimulation with nonverbal sounds containing rapid (40 msec) or extended (200 msec) frequency transitions. Bilateral symmetric activation was observed in the auditory cortex for slow frequency transitions. In contrast, left-biased asymmetry was observed in response to rapid frequency transitions due to reduced response of the right auditory cortex. These results provide direct evidence that auditory processing of rapid acoustic transitions is lateralized in the human brain. Such functional asymmetry in temporal processing is likely to contribute to language lateralization from the lowest levels of cortical processing.