We report the case of a 3 year old boy who developed cardiac arrest during general anesthesia, immediately after the subcutaneous administration of hydrogen peroxide solution. Massive pulmonary embolization was hypothesized as the postoperative ECG showed an acute heart strain. However the occurrence of transient anisocoria, associated with a paresis affecting the left arm, remained unexplained. Since brain MR did not reveal any pathological data, a direct toxicity of hydrogen peroxide on a limited area of the CNS could be assumed.