Hearts have confirmed Craig Levein was taken to hospital on Monday, but he is recovering well.
Levein was in the dugout as Hearts maintained their fine start to the Scottish Premiership season with a 1-0 win at Kilmarnock on Saturday.
Hearts have won each of their first three league games of the season, including beating champions Celtic 1-0 at home, to sit top of the table.
But the club's fans will be concerned after Hearts released a brief statement on Levein on Monday.
"Hearts can confirm that manager Craig Levein was taken to hospital this morning," the club said.
"Having received medical treatment, Craig is now recovering well in hospital.
"No further comment will be made at this time and the club asks that Craig and his family's privacy be respected, as we all wish him a speedy recovery."
Levein, 53, managed Scotland between 2009 and 2012, while he has also been in charge of clubs including Leicester City.
