We have investigated the ability of methionine sulfoxide reductase (MsrA) to maintain optimal calmodulin (CAM) function through the repair of oxidized methionines, which have been shown to accumulate within CaM in senescent brain [Gao, J., Yin, D. H., Yao, Y., Williams, T. D., and Squier, T. C. (1998) Biochemistry 37, 9536-9548]. Oxidatively modified calmodulin (CaM(ox)) isolated from senescent brain or obtained by in vitro oxidation was incubated with MsrA. This treatment restores the functional ability of CaM(ox) to activate the plasma membrane (PM) Ca-ATPase, confirming that (i) the decreased ability of CaM isolated from senescent animals to activate the PM Ca-ATPase results solely from methionine sulfoxide formation and (ii) MsrA can repair methionine sulfoxides within cytosolic proteins. We have used electrospray ionization mass spectrometry to investigate the extent and rates of methionine sulfoxide repair within CaM(ox). Upon exhaustive repair by MsrA, there remains a distribution of methionine sulfoxides within functionally reactivated CaM(ox), which varies from three to eight methionine sulfoxides. The rates of repair of methionine sulfoxides within individual tryptic fragments of CaM(ox) vary by a factor of 2, where methionine sulfoxides located within hydrophobic sequences are repaired in preference to methionines that are more solvent accessible within the native structure. However, no single methionine sulfoxide is completely repaired in all CaM oxiforms. Decreases in the α-helical content and a disruption of the tertiary structure of CaM have previously been shown to result from methionine oxidation. Repair of selected methionine sulfoxides in CaM(ox) by MsrA results in a partial refolding of the secondary structure, suggesting that MsrA repairs methionine sulfoxides within unfolded sequences until native-like structure and function are re-attained. The ability of CaM(ox) isolated from senescent brain to fully activate the PM Ca-ATPase following repair by MsrA suggests the specific activity of MsrA is insufficient to maintain CaM function in aging brain. These results are discussed in terms of the possible regulatory role MsrA may play in the modulation of CaM function and calcium homeostasis under conditions of oxidative stress.