The reactivities of SH-groups of troponin and its components were studied, using 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl). At low concentrations of Ca2+ (pCa greater than 8) one rapidly- and two slowly-reacting SH-groups of troponin I and one SH-group of troponin C slowly reacting with NBD-chloride are titrated in the whole troponin complex. When Ca2+ concentration is increased up to pCa less than 5, only one slowly-reacting and one rapidly-reacting with NBD-chloride SH-groups of troponin I are titrated. The increase of Ca2+ concentration from pCa greater than 8 to pCa less than 5 results in a change of the environment polarity for the highly reactive SH-group of troponin I in the whole troponin complex. This phenomenon may suggest that the changes in troponin C structure during Ca2+ binding somehow induce changes in that of troponin I. The half-maximal change of troponin SH-groups reactivity is found at pCa 6.8. No cooperativity for Ca2+ binding by the troponin complex is observed using SH-groups titration by NBD-Cl.