Laser diffraction patterns were investigated from enzymatically isolated, unattached myocardial cells of guinea-pig and mice. Experiments were performed at 2.5 mm Ca²⁺ and room temperature. The mean sarcomere length of resting guinea-pig and mouse myocardial cells amounted to about 1.83 μm and 1.75 μm, respectively. When paced with alternating intervals by field stimulation carefully selected ventricular cells showed transient phenomena. (1) The staircase following a rested state contraction was positive in the case of guinea-pig and negative in the case of mouse myocardial cells; (2) The rested state as compared to the steady state sarcomere shortening of guinea-pig and mouse cardiac myocytes amounted to 35% and 600%, respectively; (3) The interval strength curve of guinea-pig myocardial cells passed through a maximum which was 0.26±0.06 μm (mean±s.d.) at a pacing interval, of 2 s whereas myocardial cells of mice showed a rise of shortening with, increasing intervals reaching a maximum at the rested state (0.24±0.08 μm). Results were similar to those obtained from multicellular preparations. We conclude therefore, dynamic properties of multicellular preparations are nicely reflected at the sarcomere levels.