We report about two newborns with sudden onset of inability of mechanical ventilation due to transient chest wall rigidity after fentanyl i.v. bolus of 2 and 4 microg/kg, respectively, resulting in severe hypoxemia and secondary bradycardia. A third case developed a rigidity of the tongue after fentanyl bolus, which created some unusual difficulties in bypassing the tongue for insertion an endotracheal tube. Because of common usage of this agent for analgesia we direct attention to the possibility of fentanyl-induced muscle rigidity. We underline the necessity of a slow bolus injection to prevent this dangerous adverse effect and we recommend the administration of naloxone and/or muscle relaxants as therapy in conjunction with mechanical ventilation.