Several maleimide derivatives of potential usefulness as conformational probes were tested for reactivity toward SH groups of Ca2+, Mg2+-ATPase of sarcoplasmic reticulum. These include three fluorescent labels, N-(1-anilinonaphthyl-4)maleimide (ANM), N-(p-(2-benzimidazolyl)phenyl)maleimide (BIPM), and N-(7-dimethylamino-4-methyl-3-coumarinyl)maleimide (DACM), and a spin label, 4-maleimido-2,2,6,6-tetramethylpiperidinooxyl (MSL). These reagents also exhibit a selective reactivity toward SH groups which is similar to that of N-ethylmaleimide, although these conformational probes were somewhat more reactive than N-ethylmaleimide. Based on the above finding, procedures were devised to specifically label either one of two reactive SH groups of the ATPase, namely one highly reactive but functionally nonessential (SHN) and the other, essential for the decomposition of the E-P intermediate (SHD) [Kawakita, M., et al. (1980) J. Biochem. 87, 609-617], with any one of these conformational probes. Sarcoplasmic reticulum membranes labeled with ANM at either SHN or SHD showed a characteristic fluorescence whose intensity reversibly changed in response to the removal and readdition of Ca2+ ions in the range of 10(-6) to 10(-7) M. The change could be ascribed to a conformational change of the ATPase in response to dissociation and association of Ca2+ ions at the transport site. The Ca2+-dependent fluorescence change was quantitatively different, depending on whether the ATPase was labeled at SHN or SHD. Moreover, it was probe-specific in that BIPM and DACM fluorescence did not change in response to Ca2+. The possible significance of these observations is discussed.