Dockers forward Cam McCarthy is in hospital after an incident at the club's training on Wednesday.
An ambulance had to be called after the 25-year-old collapsed during a drill.
Freo footy manager Peter Bell said the club and doctors are trying to work out the cause of the collapse.
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"Cam was training really well with a small group of teammates in a non-contact session this morning when he suddenly collapsed on the ground," Bell said.
"He was immediately assessed by our medical staff and an ambulance was called.
"Cam was subsequently admitted to hospital, where he is undergoing further assessment and tests to determine the possible cause of the collapse and what treatment will be required.
"We will be in a position to comment further in due course.
"In the meantime, we will ensure that Cam and his family are provided with all the necessary care and support required."
Fremantle restart their season against Brisbane on June 13th, though they will base themselves in a hub on the Gold Coast from next Tuesday.
McCarthy has played 70 AFL games, 49 since moving to the Dockers from the Giants ahead of the 2017 season.
