Intrinsic alertness designates the internal (cognitive) control of wakefulness and arousal; typical tasks to assess optimal levels of intrinsic alertness are simple reaction time (RT) measurements without preceding warning stimuli. Until now, cerebral networks subserving alertness after visual and somatosensory stimulation have been reported. Studies concerning other intensity aspects of alertness like sustained attention and vigilance, on the other hand, have been performed in the auditory modality, too. In a 15O-butanol PET-activation study in 10 right-handed young healthy male volunteers an intrinsic alertness network was studied for the auditory modality. In contrast with a sensorimotor control condition we found an extended predominantly right-hemisphere network similar to those reported for other sensory modalities including frontal, cingular, inferior parietal, temporal and thalamic structures, when subjects waited for and rapidly responded to a 1000 Hz tone signal by pressing a response key with the right-hand thumb. There were, however, some differences in the topography of the frontal, temporal and thalamic activations between auditory and visual stimulation which are discussed with respect to similar results for auditory vigilance and auditory selective attention tasks reported in the literature.