Transmural multipolar electrodes, sonomicrometers implanted within the left ventricular wall, and cardiac electrical stimulation techniques were used to examine the effect of transient mechanically applied traction to the left ventricular free wall on local electrophysiological properties. Twenty-five open-chest dogs were atrially paced (cycle length 400 ms) followed by insertion of timed premature extrastimuli at left ventricular epicardial pacing sites either in the vicinity of (traction zone) or remote from (nontraction-control zone) the site of left ventricular free wall traction. Electrophysiological recordings were made before and during intermittent left ventricular free wall traction applied in late diastole (rate 25 cm/s; duration 170 ms). In 22 of 25 dogs, application of traction resulted in early local ventricular activation (mean activation advancement 64 +/- 15 ms), altered QRS morphology of the last conducted atrial drive train beat, and a relative prolongation of ventricular refractoriness in the traction zone. Conversely, in the nontraction-control zone, early activation did not occur and refractoriness was unchanged. Alterations in regional myocardial blood flow (assessed by microsphere technique) did not appear responsible for the observed changes. Furthermore, phenol interruption of local sympathetic or combined sympathetic and parasympathetic innervation or verapamil pretreatment had no impact on the mechanically induced electrophysiological changes. Thus, in normal myocardium in situ, regional abnormalities in wall motion may be associated with alterations of local ventricular activation and refractoriness, factors that in the diseased heart could lead to increased susceptibility to arrhythmias.