A low mechanical impedance strain gauge that imposed insignificant preload to the myocardial fibers was tested in vitro and in vivo. The dynamic response of the gauge to an abrupt change in length (step response) and to sinusoidal perturbation was determined. The electrical output reached 95% of maximum steady-state response within 3-5 ms after a step displacement. Frequency analysis indicated a flat response up to 80 oscillations/s. The in vivo testings of the gauges were performed on intact, working swine hearts during control and ischemic flows in a regionally perfused preparation. During control perfusion the gauges demonstrated epicardial shortening in systole and early-to-mid diastole. Relaxation was confined to late diastole. With ischemic perfusion there was a progressive loss of systolic shortening, but minimal disruption in global hemodynamics. Correlative measurements were also made with sonomicrometers positioned in subepicardial myocardium. Patterns of motion, shortening, and changes in strain were similar between the two types of gauges.