Myocardial cells isolated from 8-day chick embryos were grown in monolayer culture under conditions that produce "standard embryonic" and "adult-type" cells. These cells were subjected to electric field stimulation that had a waveshape and intensities similar to those used in clinical electric countershock procedures. Photocell mechanograms obtained before, during, and after stimulation were correlated with simultaneously measured transmembrane potentials to determine the relationship between membrane polarization and arrhythmia production that occured after the stimulus. The results of these experiments demonstrate that a predictable sequence of mechanical responses occurs after stimuli ranging in intensity from 6 to 200 V/cm. This sequence, which closely resembles that observed in vivo after similar stimulation intensities, consists of a single response (activation), tachyarrhythmia, relaxed arrest followed by transient tachyarrhythmia, arrest with contracture, and cellular fibrillation. This diverse pattern of arrhythmias is associated with a prolonged depolarization of the cell membrane which increases with the intensity of the applied stimulus. It is probable that this depolarization is caused by a transient electromechanical deformation of the cell membrane during the shock. These findings contribute to a better understanding of the causes of the arrhythmias that appear after clinical and experimental electric countershock procedures.