The sarco/endoplasmic reticulum Ca2 +-ATPase (SERCA) plays a critical role in Ca2 + homeostasis via sequestration of this ion in the sarco/endoplasmic reticulum. The activity of this pump is inhibited by oxidants and impaired in aging tissues and cardiovascular disease. We have shown previously that the myeloperoxidase (MPO)-derived oxidants HOCl and HOSCN target thiols and mediate cellular dysfunction. As SERCA contains Cys residues critical to ATPase activity, we hypothesized that HOCl and HOSCN might inhibit SERCA activity, via thiol oxidation, and increase cytosolic Ca2 + levels in human coronary artery endothelial cells (HCAEC). Exposure of sarcoplasmic reticulum vesicles to preformed or enzymatically generated HOCl and HOSCN resulted in a concentration-dependent decrease in ATPase activity; this was also inhibited by the SERCA inhibitor thapsigargin. Decomposed HOSCN and incomplete MPO enzyme systems did not decrease activity. Loss of ATPase activity occurred concurrent with oxidation of SERCA Cys residues and protein modification. Exposure of HCAEC, with or without external Ca2 +, to HOSCN or HOCl resulted in a time- and concentration-dependent increase in intracellular Ca2 + under conditions that did not result in immediate loss of cell viability. Thapsigargin, but not inhibitors of plasma membrane or mitochondrial Ca2 + pumps/channels, completely attenuated the increase in intracellular Ca2 + consistent with a critical role for SERCA in maintaining endothelial cell Ca2 + homeostasis. Angiotensin II pretreatment potentiated the effect of HOSCN at low concentrations. MPO-mediated modulation of intracellular Ca2 + levels may exacerbate endothelial dysfunction, a key early event in atherosclerosis, and be more marked in smokers because of their higher SCN− levels.