Ventricular pacing alters the pattern of electrical activation of the heart, resulting in a region of late activated tissue far from the pacing site. In late activated tissue, previous studies have described an area of mechanical stretch termed ‘prestretch’. We hypothesized that the magnitude of prestretch is related to the amount of late activated tissue. To test this hypothesis, an array of radiopaque markers was inserted into the anterior and lateral walls of the LV in 2 open chest dogs, forming a 5 by 5 cm grid. The animals underwent ventricular pacing at a remote site before and after administration of procainamide. Finite strains were determined from biplane cineradiography of the markers. Maximum prestretch was defined as the largest circumferential strain at the time of peak +dP/dt. Conduction time was defined as the time for a posterior wall stimulus to reach the anterior wall. Conduction time was an average of 81 ms before and 92 ms after procainamide, reflecting a reduced conduction velocity and an increase in late activated tissue. As the amount of late activated tissue increased from an average of 68% to 80% of the array, maximum prestretch (circumferential strain) increased from 0.28 to 0.58. In areas of prestretch, circumferential strains were, on average, 3.5 times larger than longitudinal strains, similar to previous data for passive inflation of the dog heart. Thus the magnitude of prestretch is determined by the amount of late activated tissue, and prestretch may be a continuation of passive filling mechanics in those areas of tissue not yet activated. Supported by NIH grant HL32583.