Based on studies in swine, the malignant hyperthermia syndrome has been postulated to result from an enhanced sensitivity (low threshold) of the Ca2+-induced Ca2+-release process. However, fatty acid production is elevated in homogenates of skeletal muscle from pigs and humans susceptible to malignant hyperthermia. In the present study, we demonstrate that the threshold of Ca2+-induced Ca2+ release is normal in susceptible humans and in susceptible swine depleted of triglycerides. Exogenously added unsaturated fatty acids decreased the threshold of Ca2+-induced Ca2+ release to a much greater extent in porcine and equine muscle than in human muscle. When triglyceride and free fatty acid values were reduced to about 40 and 60%, respectively, of control values, malignant hyperthermia-susceptible swine did not exhibit muscle rigidity when challenged in vivo with halothane and succinylcholine and the threshold of the Ca2+-induced Ca2+-release process in heavy sarcoplasmic reticulum fractions was normal. Despite the reduced triglyceride and fatty acid levels, these swine had a positive in vitro contracture test for malignant hyperthermia. A low Ca2+-induced Ca2+-release threshold is not essential for malignant hyperthermia susceptibility, but appears to be the result of excessive free fatty acids produced during organelle isolation.Key words: calcium release, fatty acids, malignant hyperthermia, triglycerides.