The contractile pattern of the regional left ventricular wall during premature ventricular contraction was analyzed in conscious dogs instrumented with an ultrasonic dimension gauge across the anterior and posterior left ventricular walls. Aortic flow was measured with an electromagnetic flow probe. A single premature ventricular contraction was induced by stimulating either the anterior or posterior wall with varied coupling intervals from 380 to 650 msec. Stroke volume of premature ventricular contraction was significantly smaller than that of premature atrial contraction with identical coupling intervals. In premature contractions, stroke volume was linearly related to coupling intervals. Though there was no isovolumic wall thickening in premature atrial contraction, the wall started to thicken during isovolumic ventricular systole in premature ventricular contraction. There was a clear inverse correlation between the ratio of the isovolumic wall thickening to the total wall thickening and coupling intervals. In premature ventricular contractions with identical coupling intervals, the deformation of thickening characteristics was more pronounced in regions with closer proximity to the ectopic focus. Thus it is concluded that the pump function is depressed in premature ventricular contraction, in part due to the increased ratio of wall thickening during isovolumic systole before the opening of the aortic valve. Isovolumic wall thickening increases along with the shorter coupling intervals and closer proximity to the ectopic focus. These alterations in left ventricular mechanical function due to ectopic contraction might induce serious sequelae, depending upon the ectopic focus in the presence of already depressed regional function.