Malignant hyperthermia (MH) is potentially fatal pharmacogenetic disorder of skeletal muscle caused by intracellular Ca2+ dysregulation. NCX is a bidirectional transporter that effluxes (forward mode) or influxes (reverse mode) Ca2+ depending on cellular activity. Resting intracellular calcium (Ca2+r) and sodium (Na+r) concentrations are elevated in MH susceptible (MHS) swine and murine muscles compared with their normal (MHN) counterparts, although the contribution of NCX is unclear. Lowering Na+e elevates Ca2+r in both MHN and MHS swine muscle fibers and it is prevented by removal of extracellular Ca2+ or reduced by t-tubule disruption, in both genotypes. KB-R7943, a nonselective NCX3 blocker, reduced Ca2+r in both swine and murine MHN and MHS muscle fibers at rest and decreased the magnitude of the elevation of Ca2+r observed in MHS fibers after exposure to halothane. YM-244769, a high affinity reverse mode NCX3 blocker, reduces Ca2+r in MHS muscle fibers and decreases the amplitude of Ca2+r rise triggered by halothane, but had no effect on Ca2+r in MHN muscle. In addition, YM-244769 reduced the peak and area under the curve of the Ca2+ transient elicited by high K+e and increased its rate of decay in MHS muscle fibers. siRNA knockdown of NCX3 in MHS myotubes reduced Ca2+r and the Ca2+ transient area induced by high K+e. These results demonstrate a functional NCX3 in skeletal muscle whose activity is enhanced in MHS. Moreover reverse mode NCX3 contributes to the Ca2+ transients associated with K+-induced depolarization and the halothane-triggered MH episode in MHS muscle fibers. Background: Dysregulation of Ca2+ homeostasis have been described in malignant hyperthermia (MH). Results: Na+/Ca2+ exchanger (NCX3) reverse mode activity is enhanced in MH muscles and it contributes to resting intracellular calcium concentration and Ca2+ transients induced by high K+e and by halothane. Conclusion: NCX3 reverse mode activity is increased in MH muscle. Significance: Understanding mechanisms influencing Ca2+ dynamics in MH muscle.