Six newborn cocker spaniel puppies were saved from being dumped in a carrier bag, police said.
Two Durham police officers spotted a man swinging a carrier bag on the edge of a bank near Bishop Auckland.
When they put on their blue lights, he threw the bag on to the front seat and pretended to check a tyre, they said. The puppies were found in the bag.
A man in his 50s was arrested on suspicion of animal cruelty and other offences.
The five-day-old puppies, which have been reunited with their parents, are now being looked after by vets after being found in the car on the A689 on Saturday morning.
PC Liam Vernon from Durham Police said: “The puppies were riddled with fleas and if we’d have passed just seconds before, we would have been none the wiser.
“Both myself and PC Luke Howie are huge dog lovers, so it has been one of those jobs which has really stuck with us.
“We’ve hardly stopped thinking about it but are just glad to have saved them – it really was right place, right time.”
The officers heard squealing coming from the sealed bag and found the puppies, some which had not opened their eyes, inside.
A man in his 50s was arrested on suspicion of animal cruelty, money laundering and trafficking in a controlled drug.
He has been released on bail while investigations continue.