The purpose of the present study was to describe sarcomere dynamics in thin trabeculae that were dissected from the right ventricle of kittens. The muscles were mounted in an experimental chamber and perfused with a modified Krebs-Henseleit solution (25 degrees C). Sarcomere length (SL) was measured by laser diffraction techniques; force, by a silicon strain gauge; and velocity of sarcomere shortening, by the "isovelocity release" technique. Contractile activation was varied by changing the calcium concentration in the bathing medium ([Ca2+]o). Slack SL was 1.85 +/- 0.04 microns. When length of the muscle was held constant, the sarcomeres in the central region of the muscle shortened during the twitch by up to 21%. Both peak sarcomere isometric twitch force and unloaded velocity of sarcomere shortening (Vo) were a sigmoidal function of [Ca2+]o. Maximum Vo was 9.8 +/- 0.2 microns/sec; maximum sarcomere isometric twitch force was 108 +/- 13.8 mN/mm2. The shape of the force-SL relation was a function of [Ca2+]o. At [Ca2+]o = 1.5 mM, the force-SL relation was curved with its convexity toward the abscissa; at [Ca2+]o = 6 mM, the relation was curved with its convexity toward the ordinate. Vo depended in a linear manner on SL up to 2.2 microns at [Ca2+]o = 1.5 mM. At [Ca2+]o = 6.0 mM, Vo depended also linearly on SL below 1.85 microns but became nearly independent of SL above 1.85 microns. Vo rose exponentially with time into the twitch up to 200 msec and remained constant thereafter at least up to 300 msec. Force, on the other hand, peaked at about 300 msec. These results are comparable with those found in rat myocardium, although cat myocardium is less sensitive to [Ca2+]o and twitch kinetics are significantly slower than those in rat myocardium.