We examined regional mechanics of the left ventricular free wall in naturally pumping dog hearts during β-blockade. Local systolic wall force (F) and segment length (L) were obtained with an auxotonic force gauge and an ultrasonic dimension gauge, both inserted at the equatorial level of the wall to measure F and L in the circumferential direction. Shortening velocity (−dL/dt) was obtained by differentiation of L, Preload and afterload were changed by acute caval and/or aortic occlusion so that a wide variation in F, L, -dL/dt, and dF/dt during shortening was obtained. In all experiments, F vs. L at identical time (t) after end-diastole (ED) fell on well-defined lines, Irrespective of the -dL/dt values. Control F (at t = 200 ma) over the 0.5-cm wide gauge was 85.5 ± 11.5 g, and slope of the F vs. L line (t = 200 msec) was equivalent to a drop of approximately 50% in F at 10% reduction in L. No defined relationship was observed between F, L, and -dL/dt. However, by superimposing F, L, and -dL/dt curves from contractions of high EDL and high -dL/dt on those from contractions of low EDL and low -dL/dt, and comparing F and -dL/dt at identical L and t, a slightly lower F (difference 2.23 ± 1.09 g, P ≈ 0.08) could be associated with the higher -dL/dt (difference 0.6 ± 0.1 muscle length/sec, P < 0.001). These data suggest that the F-L-t relationship is a valid descriptor of auxotonic contractions in the ventricular wall, and that the direct effect of shortening velocity on the wall force is modest.