RATIONALE: Vagus nerve stimulation (VNS) is an adjunctive treatment for patients with refractory epilepsy. In more than 30% of the patients VNS has no therapeutic effect. The goal of this study was to find an objective parameter that can be used as an indicator of effective stimulation of the vagus nerve. METHODS: The electrophysiological response to VNS was recorded from the vagus nerve, recurrent laryngeal nerve and larynx muscles. Nerve lesions and muscle relaxing agent were used to find the source of the electrophysiological response. A cuff-electrode for chronic stimulation and recording was implanted for chronic recording of the VNS-induced electrophysiological response after implantation. Dose-response curves were determined daily during a follow-up period of 2 months. RESULTS: VNS induced an electrophysiological response around 3 ms after start of the stimulation. This response was identified as a larynx compound action potential (LCMAP) LCMAP could be recorded immediately after surgery in 11/21 rats, while in the other 10/21 rats, a recovery period with an average of 25 days was required. Once the LCAMP could be recorded, the latency and overall characteristics of the doses response curves of the LCMAP remained stable during the entire follow-up period. CONCLUSIONS: In this study, we provide an objective electrophysiological parameter for vagus nerve activation. LCAMP may indicate recovery of the vagus nerve after implantation, which may help to determine when uptitration of VNS therapy can be initiated. LCAMP could be of value in future experiments for objectification of VNS in animal models for epilepsy.