Abstract The triggering of ectopic action potentials (APs) at axon terminals in a chronic epileptic cobalt focus was investigated in thalamocortical (TC) neurons of rats under urethane anesthesia. TC cells which were in register with an active epileptic aggregate discharged bursts of 2–11 APs. According to the rules of the collision test, we ascertained that bursts contained APs of ectopic and/or somatic origin. During a transient blockage of TC orthodromic discharges produced by raising the extracellular concentration of Mg 2+, ectopic bursts which were in close time relationship with the focal interictal electrocorticographic spikes persisted. These results demonstrate (i) the antidromic nature of identified ectopic APs and (ii) that, during the interictal stage, such axonal APs were not a consequence of TC discharges. The possible mechanisms for the triggering of ectopic axonal APs in the chronic cobalt focus are discussed.