The effects of stretches and releases on the contractile performance of isolated single frog atrial cells (Rana catesbeiana) were investigated. A stretch or release was imposed on the cell-either during a contraction (test) or before the onset of contraction (control)-and the contractile performance (length, velocity and force) of the test contraction was compared with that of the control contraction to determine whether the stretch or release imposed on the contracting cell altered the contractility of the cell. We found that the velocity of cell (and sarcomere) shortening for the remainder of the test contraction following either a stretch or release was markedly less than that occurring at the same time in the control contraction. This decreased velocity occurred even though the force in the test contraction was less than that in the control contraction and the sarcomere length was longer in the test contraction than in the control contraction. These results indicate that after a stretch or release imposed on the contracting cell, the force-velocity relationship at any given length and time is depressed than had the stretch or release not been imposed on the contracting cell. Thus, stretches and releases applied to the contracting single cardiac cell either produce a long-term depression in the contractility of the cell, or that the contractility at any given time and sarcomere length depends markedly on the history of the contraction.