Abstract: A pacemaker for tachycardia control which comprises a detector for sensing the onset of a tachycardia, a generator responsive to the detector for issuing a stimulating pulse to the heart for arresting the tachycardia, a second detector for sensing the response of the heart to the stimulating pulse and if the tachycardia was not arrested, for determining if the pulse was issued at an incorrect early or late time relative to the tachycardia beat, and a controller for the stimulating pulse generator responsive to the second detector whereby, when the tachycardia is not arrested, to cause a subsequent stimulating pulse to be issued at an adjusted later or earlier time relative to the tachycardia beat.