Medics, the military and the emergency services are taking part in a major exercise to see how they would cope in the evacuation of a hospital.
Around 140 people are involved in the operation at the Friarage Hospital in Northallerton, where a disused ward has been filled with smoke to simulate a fire.
Doctors, nurses, the Fire Service and Ambulance Service are working to evacuate the ward, which has been filled with hospital staff playing the roles of patients.
The exercise director says they are aiming to create as real a scenario as possible so the teams are equipped to deal with an actual emergency.
More top news
Health services that dealt with a double murderer should have identified the probability that he would re-offend
The Sunderland boss Dick Advocaat has tonight left the club.
Plans are underway to transform Auckland Castle into a major tourist destination with the help of a £10 million grant.