OBJECTIVE: The authors present preliminary findings from the first nonhuman primate neuropathological study of ECT to use perfusion fixation and adequate controls and the first to compare ECT with magnetic seizure therapy, to their knowledge. METHOD: Twelve Macaca mullata received 6 weeks of daily ECT, magnetic seizure therapy, or anesthesia alone. After perfusion fixation, their brains were examined while masked to intervention. RESULTS: No identified lesions were attributable to the interventions. Cortical and hippocampal immunoreactivity for glial fibrillary acidic protein (an astrocytic marker) was most intense in the group that received ECT. CONCLUSIONS: This small but rigorous primate study supports the view that ECT does not produce histological lesions in the brain and provides the first comparable safety data on magnetic seizure therapy.