Positron emission tomography (PET) with 18F-labelled fluorodeoxyglucose (FDG) was used to examine the regional cerebral metabolism of glucose in the limbic regions in 17 patients with schizophrenia (14, Catego Class S+ & 3, Catego Class P+) and 15 patients with mania (Catego Class M+). Also, 6 patients with psychotic depression (Catego Class D+), 4 with non-psychotic depression (Catego Class R+) and 10 healthy volunteers were imaged for comparison. Subjects were all right handed and were imaged at rest with the eyes closed and ears unplugged.A significantly increased relative uptake of FDG in the right amygdala was found in all 'psychotic' patients compared to the normal controls. In the schizophrenic patients, the significant increase above normal controls was limited to the right amygdala and right parahippocampal gyrus, whereas in the affective disorder groups there were widespread increases across other limbic structures. Significant decreases in relative FDG uptake were apparent only in the left mediofrontal cortex in the schizophrenic and manic groups in comparison to the normal controls.These results might be accounted for by an increase in dopaminergic input into limbic areas, which is generalised to most limbic regions in the affective disorders and localised to the right amygdala and right parahippocampal regions in schizophrenia. The results seem compatible with the model of psychosis proposed by Aschroft et al (1981).