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The human neuroanatomical substrate of sound intensity discrimination was investigated by combining psychoacoust ics and functional neuroimaging. Seven normal subjects were trained to detect deviant sounds presented with a slightly h igher intensity than a standard harmonic sound, using a Go/NoGo paradigm. Individual psychometric curves were carefully assessed using a three-step psychoacoustic procedure. Subjects were scanned while passively listening to the standard so und, and while discriminating changes in sound intensity at four different performance levels (d'=1.5, 2.5, 3.5 an 4.5). Analysis of rCBF data outlined activation, during the discrimination conditions, of a right hemispheric fronto-parietal network already reported in other studies of selective or sustained attention to sensory input, and in which activity a ppeared inversely proportional to intensity discriminability. Conversely, a right posterior temporal region included in secondary auditory cortex was activated during discrimination of sound intensity independently of performance level. The se findings suggest that discrimination of sound intensity involves two different cortical networks: a supramodal right fronto-parietal network responsible for allocation of sensory attentional resources, and a region of secondary auditory cortex specifically involved in sensory computation of sound intensity differences.