The effects of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) on force and intracellular Ca²⁺ transient were studied during isometric twitches and tetanuses in single frog muscle fibers. BAPTA was added to the bathing solution in its permeant AM form (50 and 100 µM). There was no clear correlation between the changes in force and the changes in Ca²⁺ transient. Thus during twitch stimulation BAPTA did not suppress the Ca²⁺ transient until the force had been reduced to <50% of its control value. At the same time, the peak myoplasmic free Ca²⁺ concentration reached during tetanic stimulation was markedly increased, whereas the force was slightly reduced by BAPTA. The effects of BAPTA were not duplicated by using another Ca²⁺ chelator, EGTA, indicating that BAPTA may act differently as a Ca²⁺ chelator. Stiffness measurements suggest that the decrease in mechanical performance in the presence of BAPTA is attributable to a reduced number of active cross bridges. The results could mean that BAPTA, under the conditions used, inhibits the binding of Ca²⁺ to troponin C resulting in a reduced state of activation of the contractile system.