Robbers snatched cash and drugs - used to sedate animals during operations - in an overnight raid at a vets.
Police say two safes had been broken into at Westway Vets, in Seaham.
An investigation is under way and security has been increased at the Church Street practice.
Durham Constabulary has not said how much money was stolen.
However, tramadol, lidocaine, adrenaline and apamorphine were among the drugs snatched.
"These drugs could prove very dangerous if they fall into the wrong hands or are unwittingly taken without a person knowing what they are," said Detective Constable John Forster.
“We would encourage anyone with information to come forward urgently before any further harm is done."
The raid happened overnight between Sunday and Monday.
Staff who turned up to work yesterday morning discovered the safes had been ransacked.
If you have any information call 101 and quote incident reference number 66 of February 17.
