A man who had his Subaru WRX stolen while his five-year-old boy was still inside said he was scared for his life but his son thought it was "just fun".

Key points: An Adelaide man advertised his Subaru WRX for sale online

An Adelaide man advertised his Subaru WRX for sale online Two people who came to see the car drove off with it while the owner's son was inside

Two people who came to see the car drove off with it while the owner's son was inside Police are looking for the pair and the car

Police said a man and a woman stole the man's car with his son inside in Adelaide's north-east last night.

They are still looking for the blue 1998 Subaru WRX, which was advertised on Gumtree.

Police said the suspects, accompanied by the car's owner and his son, were test-driving the vehicle in Hope Valley when they asked to swap drivers about 7:30pm.

Police allege the male suspect drove off in the car as the owner made his way to the front passenger's seat.

The owner's son was still sitting on the back seat, beside the female suspect.

The owner used another car to pursue the suspects, who dropped off the boy on the side of the road, unharmed, about 300 metres away.

Police are now searching for the car and the man and woman.

The car's owner, who did not want to be identified, said he was scared for his life and had difficulty sleeping last night.

"They could do anything ... if they had a gun they could shoot other people," the 30-year-old said.

"I haven't seen people like this before."

He said his wife was worried about leaving home, but his son did not appear to be scarred by the incident.

"He thought it was just fun or a joke, maybe because he's little and doesn't understand," he said.

The Subaru WRX that was stolen. ( Gumtree )

Commissioner says incident is 'concerning'

Police Commissioner Grant Stevens described the couple's behaviour as "a disgrace".

"It's very concerning and I think it goes to the nature of the people involved that they have no regard for the welfare of [the] child or the parents," he said.

The car has the registration S559APO.

The male suspect was described as Caucasian, with a medium build, bald, with numerous tattoos on both arms and legs, wearing black shorts and a white T-shirt.

The woman was described as having blonde hair and wearing a white jumper.

Motor Trade Association marketing membership manager Daniel Forbes said dealers generally took a copy of a person's drivers licence before letting them test-drive a car.

"There's a number of protections that are available to the company to stop this sort of behaviour," he said.