In addition to two published cases of malignant hyperthermia (M.H.) 13 more cases have been observed in this institution since 1971, which are probably early reactions of the M.H. type. All 13 patients were children, 3-12 years of age, who developed a rigor of the jaw muscles after succinyldicholine and/or halothane within minutes after the induction of anesthesia. Further symptoms that could be observed regularly were a tachycardia of several hours duration and a marked elevation of the serum CPK-activity with a peak after 24 hours. Since the anesthesia was terminted immediately and the operations postponed as soon as a M.H. reaction was suspected, the rise in body temperature was minimal or absent and exceeded in no case 39 degrees C. Several children were operated upon later in neuroleptanalgesia (NLA) supplemented by nitrous oxide without any muscle relaxants. None of these children developed the aforementioned symptoms rigor, techycardia and CPK-elevation. One child had Alloferin in addition to NLA-nitrous oxide-anesthesia. This child had postoperatively a tachycardia lasting several hours and 24 hours later a serum CPK activity of 1500 U/1. Since this CPK activity is higher than in comparable cases (see fig. 1) it is concluded, that the nitrous oxide-NLA sequence appears to be safe for patients susceptible to M.H. Alloferin however must probably be regarded as a trigger agent for M.H.