Derrick, Thomson and Thompson, Magnum and Poirot, all named after fictional detectives, are being groomed to help police not only keep the streets clean but also save time and money.
"As far as we know we're the first in the world to train rats to be used in police investigations," said Mark Wiebes, who heads the Dutch police "innovation" centre.
Each rat costs just €10 (B420) and can, in theory, be taught to identify any odour, from drugs to gunpowder, while a sniffer dog costs tens of thousands of euros to buy and train.
The rats have been living in a cage inside a small building since late 2011, alongside the grounds where police dogs and horses are trained.
The rats learn quickly in the darkness the nocturnal rodents prefer.
"They need barely 10 to 15 days to learn to distinguish a certain smell," said the policewoman in charge of the project, Monique Hamerslag, while lifting Derrick out of his large cage and into a smaller one where he will be put to work.
While getting fingered by a rat cannot be used in court, it gives forensics a clue as to what they should test first. Dutch police can only hold someone without evidence for a few hours, so the rats can help prevent a guilty suspect being released.
Hamerslag developed the idea in her thesis while studying to join the police, drawing inspiration from an aid group that used rats to sniff out anti-personnel mines in Tanzania.
Police rats will never replace police dogs, she said, but they have different and complimentary skill sets.
"A dog is a natural predator, and so is not afraid to explore an unfamiliar place," said Wiebes. "On the other hand a dog couldn't live like a rat, spending their whole time in a small enclosed space, he'd get bored."