A major incident at the Aura Leisure Centre in Letterkenny took place on Saturday morning.
There appears to have been an issue in relation to water in the swimming pool and a number of children took a reaction to it.
It has emerged that around 50 children were in the pool.
Up to six children were taken to Letterkenny University Hospital as a precautionary measure.
At least four ambulances were outside the complex around midday. Fire brigade personnel were also in attendance.
Gardai were on duty at the entrance and admissions were halted.
Donegal councillor Gerry McMonagle told Newstalk that some children had "some sort of allergic reaction in the pool".
And he added: "There was about a dozen ambulances and fire tenders and the Gardaí.
"From asking and inquiring as to what happened, I was told there had been some sort of allergic reaction in the pool for the children - who were quickly evacuated by the Aura staff to a safe place and emergency services contacted."
"A number of children have become sick while they were swimming, and it seems there was some sort of reaction in the pool area."
There have also been reports of a dark liquid entering the water.
Letterkenny University Hospital put its emergency plan in place.
A full investigation, involving a number of agencies, is now expected to get underway.
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