Reports in vitro experiments on the source(s) of the magnetic fields produced by the brain. Theoretical arguments suggest that the dominant sources should be dipolar and oriented parallel to the scalp. Using an isolated turtle cerebellum as a model, the authors find that the fields produced following dorsal stimulation are attributable to current flow perpendicular to the cerebellum surface, suggesting Purkinje cell sources. The authors also discuss observations of longer lasting fields associated with spreading depression induced in the cerebellum.