Emergency services from across the East Midlands will be testing their readiness for dealing with major terrorist attack on Sunday.
The day-long exercise in Northampton will be based on a theoretical "dirty bomb" attack.
It will be rehearsing how to deal with a chemical, biological, radiological and nuclear incident.
The operation will test how well the police, fire and ambulance services work with each other.
Emergency services from Derbyshire and Leicestershire will be joining the exercise, organised by Northamptonshire County Council's emergency planning team.
Staff at Northampton General Hospital will be also be testing its procedures.
Northamptonshire fire divisional officer Ian Watson said while there was no specific threat to the area, it paid to be prepared for any kind of situation.
The attack will be simulated at Northampton Saints' rugby ground.