Nitric oxide (NO) physiologically stimulates the sarco/endoplasmic reticulum calcium (Ca²⁺) ATPase (SERCA) to decrease intracellular Ca²⁺ concentration and relax cardiac, skeletal and vascular smooth muscle. Here, we show that NO-derived peroxynitrite (ONOO⁻) directly increases SERCA activity by S-glutathiolation and that this modification of SERCA is blocked by irreversible oxidation of the relevant cysteine thiols during atherosclerosis. Purified SERCA was S-glutathiolated by ONOO⁻ and the increase in Ca²⁺-uptake activity of SERCA reconstituted in phospholipid vesicles required the presence of glutathione. Mutation of the SERCA-reactive Cys674 to serine abolished these effects. Because superoxide scavengers decreased S-glutathiolation of SERCA and arterial relaxation by NO, ONOO⁻ is implicated as the intracellular mediator. NO-dependent relaxation as well as S-glutathiolation and activation of SERCA were decreased by atherosclerosis and Cys674 was found to be oxidized to sulfonic acid. Thus, irreversible oxidation of key thiol(s) in disease impairs NO-induced relaxation by preventing reversible S-glutathiolation and activation of SERCA by NO/ONOO⁻.