To investigate mechanisms of audio-vocal interactions in the human brain, we studied the effect of speech output on modulation of neuronal activity in the auditory cortex. The modulation was assessed indirectly by measuring changes in cerebral blood flow (CBF) during unvoiced speech (whispering). Using positron emission tomography (PET), CBF was measured in eight volunteers as they uttered syllables at each of seven rates (30, 50, 70, 90, 110, 130 or 150/min) during each of the seven 60-s PET scans. Low-intensity white noise was used throughout scanning to mask auditory input contingent on the whispering. We found that, as a function of the increasing syllable rate, CBF increased in the left primary face area, the upper pons, the left planum temporale and the left posterior perisylvian cortex. The latter two regions contain secondary auditory cortex and previously have been implicated in the processing of speech sounds. We conclude that, in the absence of speech-contingent auditory input, the modulation of CBF in the auditory cortex is mediated by motor-to-sensory discharges. As such, it extends our previous findings of oculomotor corollary discharges to the audio-vocal domain.