Interneuron progenitors from the embryonic medial ganglionic eminence (MGE) can migrate, differentiate, and enhance local inhibition after transplantation into the postnatal cortex. Whether grafted MGE cells can reduce ictal activity in adult neocortex is unknown. We transplanted live MGE or killed cells (control) from pan green fluorescent protein expressing mice into adult mouse sensorimotor cortex. One week, 2 and 1/2 weeks, or 6 to 8 weeks after transplant, acute focal ictal epileptiform discharges were induced by injection of 4-aminopyridine (4-AP) 2 mm away from the site of transplantation. The local field potential of the events was recorded with 2 electrodes, 1 located in the 4-AP focus and the other 1 in the transplantation site. In all control groups and in the 1-week live cell transplant, 4-AP ictal discharges revealed no attenuation in power and duration from the onset site to the site of transplantation. However, 2.5 or 6 ~ 8 weeks after MGE transplants, there was a dramatic decrease in local field potential power at the MGE transplanted site with little decrease in ictal duration. Surprisingly, there was no relationship between grafted cell distribution or density and the degree of attenuation. As remarkably low graft densities still significantly reduced discharge power, these data provide further support for the therapeutic potential of interneuron precursor transplants in the treatment of neocortical epilepsy.