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Thirty-three patients, with normal muscle function, ranging in age from 7-82 years and undergoing elective orthopedic surgery agreed to quadriceps muscle biopsy. At all ages, contracture responses to caffeine or to 2% halothane and caffeine at 37°C were similar. The threshold to caffeine was 2-4 mM, and the plateau of the dose-response curve was above 32 mM; the threshold to halothane-caffeine was 0.25-0.5 mM, and the plateau 16 mM. Contractures were not observed when the muscle bundles were exposed solely to 2% halothane. Incubation of muscle bundles with 2% halothane for 45 min at 37°C resulted in minimal depletion of energy stores. The ratio of the halothane-exposed concentration to that of the non- halothane level was, for adenosine triphosphate and creatine phosphate, respectively, about 1.0, and 0.9. Examination of contracture responses did not reveal changes with maturation or aging, and do not explain age-related changes in muscle function.