Malignant hyperthermia (MH) susceptibility has been attributed to a leaky sarcoplasmic reticulum (SR) caused by missense mutations in RYR1 or CACNA1S, and the MH crisis has been attributed solely to massive self-sustaining release of Ca 2+ from SR stores elicited by triggering agents. Here, we show in muscle cells from MH-RyR1 R163C knock-in mice that increased passive SR Ca 2+ leak causes an enlarged basal influx of sarcolemmal Ca 2+ that results in chronically elevated myoplasmic free Ca 2+ concentration ([Ca 2+] i) at rest. We discovered that Gd+ 3 and GsMTx-4 were more effective than BTP2 or expression of the dominant-negative Orai1 E190Q in reducing both Ca 2+ entry and [Ca 2+] i, implicating a non-STIM1/Orai1 SOCE pathway in resetting resting [Ca 2+] i. Indeed, two nonselective cationic channels, TRPC3 and TRPC6, are overexpressed, and [Na] i is chronically elevated in MH-RyR1 R163C muscle cells.[Ca 2+] i and [Na+] i are persistently elevated in vivo and further increased by halothane in MH-RyR1 R163C/WT muscle. These increases are markedly attenuated by local perfusion of Gd+ 3 or GsMTx-4 and completely suppressed by dantrolene. These results contribute a new paradigm for understanding MH pathophysiology by demonstrating that nonselective sarcolemmal cation channel activity plays a critical role in causing myoplasmic Ca 2+ and Na+ overload both at rest and during the MH crisis.—Eltit, JM, Ding, X., Pessah, IN, Allen, PD, Lopez, JR Nonspecific sarcolemmal cation channels are critical for the pathogenesis of malignant hyperthermia.