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, J. M., Ding, X., Pessah, I. N., Allen, P. D., Lopez, J. R. Nonspecific sarcolemmal cation channels are critical for the pathogenesis of malignant hyperthermia.