Twelve patients with catatonic schizophrenia and 12 matched healthy controls were examined with functional MRI while performing a motor task. The aim of our study was to identify the intracerebral pathophysiological correlates of motor symptoms in catatonic patients. The motor task included three conditions: a self-initiated (SI), an externally triggered (ET) and a rest condition. Statistical analysis was performed with SPM5. During the self-initiated movements patients showed significantly less activation than healthy controls in the supplementary motor area (SMA), the prefrontal and parietal cortex. Our results suggest a dysfunction of the "medial motor system" in catatonic patients. Self-initiated and externally triggered movements are mediated by different motor loops. The "medial loop" includes the SMA, thalamus and basal ganglia, and is necessary for self-initiated movements. The "lateral loop" includes parts of the cerebellum, lateral premotor cortex, thalamus and parietal association areas. It is involved in the execution of externally triggered movements. Our findings are in agreement with earlier behavioral data, which show deficits in self-initiated movements in catatonic patients but no impairment of externally triggered movements.