Masseter (jaw) muscle rigidity (MMR) denotes trismus to the extent that it is difficult or impossible to open the jaw. Masseter spasm has a spectrum of severity from mild increase in jaw tension to “jaws of steel.” A patient who exhibits marked rigidity of the jaw muscles should not be discharged. Overnight observation for temperature rise, myoglobinuria, elevated CK levels or progression to an MH episode is required. Patients who experience milder increases in jaw tension should be observed for signs and symptoms of MH for at least 12 hours. If there is evidence of myoglobinuria, dark cola-colored urine, increase in temperature, pulse rate, and abnormality of acid base balance, the patient should be admitted and observed overnight.