Positron emission tomography was used to investigate cerebral blood flow (CBF) changes associated with the processing of speech. In a first experiment, normal right-handed volunteers were scanned under two conditions that required phonetic processing (discrimination of final consonants and phoneme monitoring), and one baseline condition of passive listening. Analysis was carried out by paired-image subtraction, with MRI overlay for anatomical localization. Comparison of each phonetic condition with the baseline condition revealed increased CBF in the left frontal lobe, close to the border between Broca's area and the motor cortex, and in a left parietal region. A second experiment showed that this area was not activated by a semantic judgement task. Reanalysis of data from an earlier study, in which various baseline conditions were used, confirmed that this region of left frontal cortex is consistently involved in phonetic tasks. The findings support a model whereby articulatory processes involving a portion of Broca's area are important when phonetic segments must be extracted and manipulated, whereas left posterior temporal cortex is involved in perceptual analysis of speech.