We tested if interaction between the thick and thin filaments, as revealed by the rate of cross-bridge cycling or the pattern of calcium activation, influences the relation between tension and free calcium concentration (pCa) of cardiac muscle. Bundles of rabbit ventricular cells which contained either fast-cycling RV 1 myosin or slowly-cycling RV 3 myosin and bundles of human atrial cells were chemically skinned by exposure to saponin. Skinned bundles were rapidly activated with calcium using the method of Moisescu and Thieleczek (1978). Our results suggest that the relation between pCa and tension is not different for skinned bundles of rabbit ventricular cells that contain either fast or slowly cycling myosin cross-bridges. The pattern of calcium activation, i.e. maximal to submaximal or submaximal to maximal, was also found not to influence the relation between tension and pCa for either rabbit ventricular or human atrial muscle. The tension-pCa relation of cardiac muscle was not altered by conditions that have been shown to alter the tension-pCa relation of skeletal muscle. These results suggest that the mechanisms responsible for regulating force development at the level of the thick and thin filaments that have been reported for skeletal muscle, are absent in cardiac muscle.