Abstract Epileptogenic foci were produced by iontophoresis of cobaltous chloride onto cat cortex. The epileptiform discharge, as recorded with a surface electroencephalogram electrode, was characterized by an initial large amplitude spike followed by prominent afterdischarge. The focus was restricted in its lateral extent and appeared to be confined pricipally to the superficial layers of cortex. Most cells recorded in the upper half of the cortex showed excitatory patterns of activity correlated with surface epileptiform events; deeper in the cortex, increasing numbers of cells were inhibited or not affected during the surface discharge. The maximum duration of spontaneous epileptogenesis induced by cobalt iontophoresis was 24 hr.