On the basis of isotopic methods it has been found that force generation promotes increased Ca2+ binding to troponin C in cardiac muscle [P. Hofmann and F. Fuchs. Am. J. Physiol. 253 (Cell Physiol. 22): C541-C546, 1987] but not in skeletal muscle (J. Muscle Res. Cell Motil. 6: 477, 1985). However, studies with skinned rabbit psoas muscle fibers containing substituted fluorescent troponin C analogues indicate that force-generating cross bridges do promote increased Ca2+ binding in skeletal muscle (K. Güth and J. D. Potter. J. Biol. Chem. 262: 13627-13635, 1987). We have reexamined this question using a modified contraction-relaxation protocol in which Ca2+ binding to detergent-treated rabbit psoas fibers was measured either during steady-state force development or after relaxation was induced by one of two myosin ATPase inhibitors, vanadate or 2,3-butanedione monoxime. A standard double-isotope technique was used to measure Ca2+ binding. Another set of experiments was done in which force was reduced by releasing muscle fibers from sarcomere lengths of 2.4-2.6 microns to 1.5-1.7 microns, and bound Ca2+ was determined either before or after the release. No statistically significant effect of force generation or sarcomere length on Ca(2+)-troponin C affinity was observed. Thus the discrepancy remains between results obtained with isotopic and fluorescence methods. It is possible that in skinned fibers emission from fluorescence probes is more closely related to protein-protein interactions than to the amount of Ca2+ bound to troponin C.