Ten mongrel dogs were used to assess regional myocardial function in response to electrical pacing from the inflow (RVI), outflow tracts (RVO), and apex (RVA) of the right ventricle and apex (LVA) and lateral wall (LVL) of the left ventricle. Straingauge arches were sutured to the epicardial segments of the apical and lateral walls of the left ventricle and their mechanical shortening was measured during right atrial (RA) and during ventricular pacing. The onset of shortening of two segments did not differ significantly in RA and RVI pacing, while the stimulation of RVA and LVA caused the initial contraction of the apical segment, and RVO and LVL pacing caused that of the lateral segment; i.e. RVA and RVO pacing resulted in left ventricular asynchrony of contraction similar to LVA and LVL pacing, respectively.Asynchrony of contraction was not accompanied by a uniform change in peak tension of regional myocardial segments, but preejection tension rose and ejection tension fell at the pacing site. The reduction in mean aortic pressure inversely correlated with the prolongation of the time interval between the onset of shortening of two segments during left ventricular pacing. The decrease in cardiac performance observed during ventricular pacing was related to the severity of asynchrony rather than the direction of the ventricular depolarization or change in regional myocardial tension. regional myocardial contraction; straingauge arch; ventricular pacing; ventricular asynchrony; cardiac performance Decreased cardiac performance during ventricular pacing has been ascribed to the lack of synchrony resulting from altered ventricular activation and contraction (Wiggers 1925;Gilmore et al. 1963;Finney 1965; Miyazawa et al. 1976). This is due to slow passage of the depolarization wave through ventricular muscle, as compared with the relatively rapid activation through the normal conduction system. However, there have been few reports concerning the regional myocardial function under this condition (Osadjan and Randall 1964;Finney 1965;Hood et al. 1969). From this standpoint , two strain gauge arches were stitched to the epicardial surface of the apical and lateral walls of the left ventricle. Tension curves in each of the segments were analyzed in control period during RA pacing