We report the case of a 29-years-old woman with Menière's syndrome undergoing surgery to drain the endolymphatic sack. After anesthetic induction trismus developed but tubes were inserted without difficulty. Tachycardia began 15 min after start of balanced anesthesia with isoflurane, as arterial oxygen saturation and CO2 pressure decreased and temperature increased. These symptoms responded favorably to immediate treatment with dantrolene sodium, which was continued for 30 h in the ward for postoperative intensive care. Blood chemistry reports showed high enzyme levels that peaked at 24 h. We stress the importance of monitoring end tidal CO2 pressure and of rapidly starting specific treatment with dantrolene for correction of the clinical picture described.