The invention provides an electrical stimulus generator adapted for implantation in a patient suffering from epilepsy, to provide electrical stimuli for application to a selected cranial nerve as a therapy to alleviate a seizure, the stimulus generator comprising: electronics effective when energized for generating said electrical stimuli； an electrically conductive case housing said electronics； and electrode means electrically connected to said electronics for sensing cardiac activity of the patient, said electronics including means responsive to sudden time rate of change of the patient's heart rate in the sensed cardiac activity for distinguishing a change which is a precursor of an imminent epileptic seizure from a change attributable to normal physical exercise, to energize the electronics to generate said electrical stimuli upon the seizure-indicative change.
