Hearts manager Craig Levein was taken to hospital on Monday morning but is recovering after receiving treatment, the club said.
It did not disclose in a statement the nature of Levein's illness or when he was likely to return.
Levein, 53, watched Hearts beat Kilmarnock 1-0 in the Scottish Premiership on Saturday for their third win in their first three games.
The former Scotland manager returned to the job on a three-year contract after a spell as director of football following the departure of Ian Cathro in August 2017.
According to the BBC, assistant Austin MacPhee will take over until Levein returns.