The effects of electrical stimulation of the vagus nerve, a proposed treatment for patients with intractable epilepsy, on focal interictal spikes produced by penicillin and EEG secondarily generalized seizures induced by pentylenetetrazol were assessed in rats. Interictal spike frequency was reduced by 33% during 20 s of stimulation (p < 0.001) and remained low for < or = 3 min. Amplitude of residual spikes was also decreased. Cardiac and respiratory rates were suppressed. Cooling the nerve proximal to the point of stimulation abolished the EEG and respiratory effects. A similar reduction in spike frequency of 39% was obtained by heating the animals' tail (p < 0.01). Vagal stimulation at onset of seizures reduced mean seizure duration from 30.2 +/- 15.7 s without stimulation to 5.0 +/- 1.8 s (p < 0.01). Only the EEG equivalent of the clonic phase of the seizure was affected. These findings suggest that vagus nerve stimulation can be a potent but nonspecific method to reduce cortical epileptiform activity, probably through an indirect effect mediated by the reticular activating system.