The neural basis of ambivalence has not yet been identified. We investigated the prefrontal cortical activations implicated in evaluative processing of ambivalent stimuli under the forced and non-forced response conditions. Cerebral blood flow was measured using H(2)(15)O positron emission tomography in twelve normal volunteers during a modified word-stem completion task that was designed to evoke different conditions of ambivalence. The prefrontal cortical activations were restricted to the orbitofrontal cortex during the non-forced ambivalent condition, whereas the ventrolateral prefrontal cortex and the frontopolar cortex were activated in addition to the orbitofrontal cortex during the forced ambivalent condition. It is remarkable that the orbitofrontal cortex and the ventrolateral prefrontal cortex demonstrated a reciprocal activation pattern, which might be linked to the evaluative attitude toward the ambivalent stimuli.