Preliminary studies suggest that rTMS is effective in treating refractory depression. In the course of treating a patient, we obtained serial fluorodeoxyglucose PET scans in order to show the effect of rTMS on cerebral metabolism. 20 Hz rTMS was administered daily to the left prefrontal area at 80% of hand muscle motor threshold for 2 seconds every minute for 20 minutes. Three scans were done while the patient performed an auditory vigilance task requiring a button press with the right hand in response to an oddball tone: one before treatment while severely depressed, and two after a successful four week course of rTMS. In the second of these, FDG was administered during the 20 minutes of cyclic rTMS. The scans were coregistered and the two post-treatment scans subtracted. Prior to treatment, cerebral metabolism (CMR-glu) was globally decreased. After treatment, global CMRglu was improved. During rTMS, CMRglu was again increased compared with the depressed state and possibly compared with the unstimulated post-treatment scan as well, but the increase was less intense in the region where rTMS was applied. This study shows that rTMS can produce different chronic and acute effects on brain function and that the therapeutic action of rTMS in depression may involve reversible depression of brain activity at the stimulation site.