Forty-four dogs were found in two vehicles after police were called by a member of the public who became suspicious.
Officers were called to Bordesley Lane in Redditch, Worcestershire, where they found a vehicle containing dogs and puppies, as well as cash.
More dogs and puppies were found in a second vehicle shortly after.
Four men have been arrested on suspicion of dog thefts and animal cruelty offences.
The force said the dogs were now at rescue centres, vet surgeries and RSPCA centres where they are being assessed and scanned for microchips.
The animals will stay at the centres while investigations are carried out.
Det Ch Insp Joanne Wood said they suspected some of the dogs had been stolen.
“At this moment in time we are not in a position to release details about each individual dog until they have all been assessed and scanned for microchips,” she said.
“We ask for the public’s patience whilst this takes place and if any of these dogs are identified as stolen, owners will of course be contacted in due course.”
The four men been released on police bail while inquiries continue.
Anyone wanted to buy a dog should be “extra vigilant” and make sure they are buying from a legitimate breeder or rescue centre, police said.