1. We measured the change in intracellular Ca2+ transients, using aequorin, in response to muscle length change during twitch contraction in ferret ventricular muscles. 2. Intracellular Ca2+ concentration ([Ca2+]i) was transiently increased when the muscle length was quickly shortened to 92% of maximum length (Lmax) at various times after stimulation (this increase in [Ca2+] is termed extra-Ca2+). The magnitude of extra-Ca2+, measured at different extracellular Ca2+ concentrations ([Ca2+]o), showed a dependence upon the magnitude of tension reduction and upon [Ca2+]i immediately before the length change. 3. In the presence of caffeine (5 mM), the difference between the Ca2+ transient at Lmax and at shorter lengths showed a time course similar to the difference between the developed tension at both lengths. A quick release in the caffeine-treated preparation produced the extra-Ca2+ with a slower time course compared with that observed in the absence of caffeine. Stretching the muscle from 96% Lmax to Lmax produced more active tension and decreased [Ca2+]i. 4. These results indicate that the affinity of troponin-C, a major Ca2+ binding protein, which controls contraction, is influenced by developed tension i.e. cross-bridge attachment and detachment.