AbstractTightly coupled mitochondria from M. longissimus dorsi (LD) muscle of different breeds of pigs showed variations in the rate of Ca2+ efflux during anaerobiosis. The initial fast efflux rate of Ca2+ could be used to relate the stress‐susceptibility of the pigs used. Mitochondria from LD muscle of stress‐susceptible (SS) Pietrain and Poland China pigs showed an anaerobic rate of Ca2+ efflux twice that of stress‐resistant (SR) Large Whites. Mitochondria from crossbreds (Large White x Saddleback) showed a Ca2+ efflux rate intermediate between that of purebred SS and SR pigs. Halothane enhanced the rate of Ca2+ efflux only in SS pigs, and this effect could be eliminated by MgCl2. The marked difference in the rate of Ca2+ efflux between SS and SR pigs, and the enhancement in the rate of Ca2+ efflux by halothane only in SS pigs, offers an ultimate explanation for the formation of PSE (pale, soft and exudative) muscle and malignant hyperthermia in SS pigs. The excess Ca2+ liberated from mitochondria of SS pigs is postulated to be the trigger for PSE meat and also the case of malignant hyperthermia.