We used ultrasonic segment length gauges to examine the regional behavior of midwall fibers at different sites around the left ventricular minor axis in 12 anesthetized dogs. In six dogs (group I) with circumferentially oriented midwall gauges, significantly greater shortening of anterior than lateral or posterior wall segments was demonstrated over a range of left ventricular end-diastolic pressures from 2 to 18 mm Hg. Normalized end-diastolic segment lengths increased more in the anterior wall as end-diastolic pressure increased, suggesting that regional differences in diastolic distensibility may in part account for the observed shortening differences. To examine the extent to which shortening of longitudinally oriented fibers of the subendocardium and subepicardium might influence the behavior of the midwall circumferential fibers, we implanted mutually perpendicular midwall gauges circumferentially and longitudinally in the anterior and posterior walls in six dogs (group II). Longitudinal shortening of midwall fibers was negligible at low end-diastolic pressures, but increased significantly at higher end-diastolic pressures. In the anterior wall, there was greater circumferential than longitudinal shortening, whereas, in the posterior wall, shortening was similar in the two directions. Finally, we calculated the midwall area subtended by the mutually perpendicular gauges and found the systolic change in midwall area to be similar for the anterior and posterior walls at all end-diastolic pressures. We conclude that midwall fibers demonstrate considerable nonuniformity of contraction at different sites around the minor axis. This finding may be related in part to regional differences in diastolic distensibility or in functional interactions between fiber layers. Despite these complex regional, directional, and volume dependent differences in midwall segment function, the systolic changes in midwall area did not vary regionally. Thus, different midwall sites around the minor axis circumference appear to have similar overall contributions to the ejection of blood.