The incident happened earlier this afternoon at Dreenan Bridge Ballybofey.
Gardaí were manning a checkpoint in the area earlier and the motorist was one of a number checked by officers for NCT and road tax.
His car was seized and was driven from the scene by Gardaí.
The man then collapsed at the side of the road and local workmen went to his rescue.
An ambulance was called and the man – a well-known and respected businessman with a history of heart complaints – was rushed to Letterkenny General Hospital where his condition is understood to be stable.
It’s understood an investigation is now underway into the circumstances surrounding the incident.
An eye witness told Donegal Daily the man was in a very distressed state.
“There were a few men working in a field nearby and they saw the man rolling around in the grass,” he said.
“They went to his aid and saw that he was in a very distressed state and immediately made contact with the emergency services.
“Thankfully their response was prompt, although it took them quite a while to remove the man from the scene as he was gravely ill and they were worried about moving him.”
There has been a huge increase in both Garda and Customs checkpoints in Donegal in recent weeks.
Last week we reported how 39 vehicles had been seized for failing to pay VRT.
Meanwhile Revenue has dismissed as “a false rumour” claims that a car was rammed during one operation in Inishowen.