Atrial and ventricular systoles were controlled electronically in non-thoracotomized dogs with acute heart block. Videodensitometer analysis of selective left ventricular videoangiograms showed presystolic reflux to the left atrium during regular cardiac driving with long atrial-ventricular intervals. Atriogenic mitral valve closure after relatively isolated atrial systole was not as effective as closure either by optimally timed atrial and ventricular systoles or by ventricular systole alone.