Life-saving medical equipment was stolen from an East Midlands ambulance as it attended an emergency call.
A defibrillator was taken as crews responded to emergencies around south Nottinghamshire and Derbyshire on Sunday.
Police said it was unclear when the equipment was taken but it is thought thieves spotted an opportunity when the crew were away from the vehicle.
East Midlands Ambulance Service has appealed for help to locate the kit.
It said the equipment would cost "many thousands of pounds" to replace.
PC Karl Barlow, of Nottinghamshire Police, said: "At the start of each shift the ambulance crew fill their vehicle with essential equipment, which includes a defibrillator.
"At some point during the crews shift the defibrillator was stolen.
"Any theft is serious, however stealing a life-saving piece of equipment from an ambulance is a grave offence, which could have had fatal consequences had the paramedics needed to use it and it had been stolen."