Regional cerebral blood flow was measured with positron emission tomography during the performance of tasks that required cognitive spatial transformations of alphanumeric stimuli. In the mirror image task, the subjects were required to discriminate between the normal and the mirror images of alphanumeric stimuli presented in the upright orientation. In the mental rotation task, the same judgement was required, but now the stimuli were presented in various orientations other than the upright one. The subjects therefore had to rotate the stimuli, in mind, into the upright position before making their decision. In relation to the control task, which involved discrimination of these same stimuli but not any form of mental transformation, there were significant increases in the right postero-superior parietal cortex and the left inferior parietal cortex in both experimental tasks. For mental rotation, specific activity was seen only within the left inferior parietal region and the right head of the caudate nucleus. These results specified the parietal areas involved in a purely cognitive spatial process and demonstrated a close interaction between these areas and the anterior neostriatum and certain lateral frontal cortical areas in the discrimination of rotated forms of stimuli.