More than 90 dogs - some of them dead - have been found in "poor conditions" in Blaenau Gwent, police said.
Officers visited a property in Tredegar with the RSPCA at 19:00 BST on Tuesday in a proceeds-of-crime inquiry.
A 42-year-old woman from the town was arrested on suspicion of acquiring, using, or possessing criminal property and causing unnecessary suffering to a protected animal.
Gwent Police said she had been released while inquiries continued.
"Our investigation is ongoing and we are assisting the RSPCA and Trading Standards in their enquires," a spokeswoman for the force said.
An RSPCA spokesman added: "A number of dogs have been removed from the property and placed into our care pending enquiries.
"This is an ongoing incident and it would not be appropriate for us to comment further at this time." he told the Local Democracy Reporting Service.