Muscle relaxation rates in individuals susceptible to malignant hyperthermia. Muscle relaxation rate following a tetanic stimulus of adductor pollicis muscle was measured prospectively in 26 patients potentially susceptible to malignant hyperthermia (MH) the day before a muscle biopsy was obtained for MH in vitro screening. Eleven subjects were found to be MH susceptible (MHS) and 15 subjects MH-negative (MHN). In all patients, relaxation rate was recorded at three different temperatures of the skin overlying adductor pollicis (30, 34 and 38 degrees C) achieved by a small surface heating unit placed over the thenar eminence. The MHS group exhibited slightly higher relaxation rate at 34 and 38 degrees C compared with the MHN group and this difference was accentuated with increasing temperature, but was not statistically different. The results of the present study suggest that relaxation rates are normal in MHS individuals under physiological conditions and cannot be used diagnostically for MH screening. 