Vesicular fragments of sarcoplasmic reticulum (SR) were labeled with the --SH-directed spin label 2,2,6,6-tetra-methyl,4-amino(N-iodoacetamide). Colorimetric titrations of the remaining --SH residues and determinations of unbound spin label indicated that primarily 3 residues/enzyme molecule were labeled under saturating conditions. This labeling was accompanied by minimal losses in activity, providing precautions were taken to prevent sulfhydryl oxidation during the labeling process. Additions of ATP produced a new "highly constrained" component in the ESR spectrum of the labeled SR, an effect not noted in previous studies. It is demonstrated that the changes produced by ATP are reversible, and require both substrate binding and Ca2+ binding. However, hydrolysis of the substrate is not required. It is further demonstrated that the labeled residue(s) responsible for the spectral change is not in the immediate vicinity of the ATP binding site. It is apparent that the observed spectral change is related to a conformational effect of ATP and Ca2+ on the ATPase protein, which is associated with a large free energy change occurring on binding. It is also suggested that the conformational effect extends to a significant distance from the nucleotide binding site and may be a precursory step to Ca2+ translocation.