Simultaneous measurement of magnetic and electrical activity in vitro hippocampal slices has been performed. A magnetic signal of 300-400 fT was observed coincident with extracellular electrical potential activity. Comparison is made with a current dipole model similar to that used for the analysis of magnetoencephalographic data, but with additional constraints obtained from the electrical signal. Limitations of this model for the hippocampal slice are discussed. © 1988.