Troponin C occurs as two isoforms, one (sTnC) expressed in fast skeletal muscle and the other (cTnC) expressed in cardiac and slow skeletal muscle. On the basis of subunit exchange experiments it has been suggested that cTnC may play a specific role as a length-sensing molecule. In this study we have compared skinned fibers from bovine ventricle and slow rabbit soleus muscle with respect to the effects of force and sarcomere length on Ca2+ binding to troponin C. A double-isotope technique was used to measure Ca2+ binding concurrent with force generation. The phosphate analogue vanadate was used to regulate force independent of free Ca2+ concentration. To determine the effect of sarcomere length, muscle fibers were released from longer sarcomere length to shorter sarcomere length, and bound Ca2+ was determined either before or after the release. Reduction in force or length was associated with reduced binding of Ca2+ to cTnC in cardiac muscle, but no effect of these interventions was seen in soleus muscle. Thus the nature of the mechanical feedback on the regulatory Ca(2+)-binding sites appears to be a property of the myofilament system rather than the troponin C isoform.