Potentially dangerous pharmaceutical drugs were stolen from an ambulance while the crew were out of the vehicle, Bedfordshire police said.
The driver and a colleague travelled from Bedford to Papworth Hospital in Cambridge on Saturday evening.
They returned to Bedford later and realised a pack of drugs was missing.
It is not known if the drugs were taken while the ambulance was at Papworth Hospital or later in Bedford but police said they could be highly dangerous.
Police listed some of the missing drugs as adrenalin, aspirin, diazepam, pain relief spray, hydrocortisone and other unspecified pharmaceuticals.