We studied sarcomere performance in single isolated intact cardiac cells using techniques that allow direct measurement of sarcomere length and force. This investigation dealt primarily with sarcomere performance during twitch contractions under lightly loaded conditions. In such contractions, there was a significant portion of the contraction in which sarcomere shortening occurred at constant velocity over a significant range of sarcomere lengths. The constant velocity phase of shortening was followed by a phase of shortening in which sarcomere velocity decreased markedly. Both the velocity and extent of sarcomere shortening depended on the stimulus parameters used to excite the cell. With threshold stimulation, sarcomere velocities during the constant velocity phase of shortening ranged from 1 to 5.5 micron/sec in different cells and significant slowing of sarcomere shortening began at sarcomere lengths of 1.8-2.0 micron. In contrast, when cells were stimulated with a long duration stimulus (200 msec) of large current strength, sarcomere velocities during the constant velocity phase ranged from 6 to 12 micron/sec, and significant slowing did not occur until a sarcomere length of about 1.6 micron was reached. The threshold stimulus strength-stimulus duration relationship was determined on the single cell, and it was found to be of the type expected for a cell having an intact excitable membrane capable of generating an action potential when depolarized to a fixed voltage threshold. The data presented in this paper give direct evidence that the lightly loaded cardiac sarcomere has a velocity of shortening which depends on the level of contractile activation but is independent of sarcomere length at sarcomere lengths greater than about 1.6 micron.