Daley began feeling off Wednesday afternoon, he was in his downtown home with his eldest daughter, Nora Daley Conroy.
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He remained at Northwestern Wednesday evening, where he was "talking and alert" and in a good mood, said his former mayoral press secretary, Jacquelyn Heard, who still works with Daley at a law firm.
His spokesperson said as of Thursday afternoon he was resting after undergoing tests. He will spend another night in the hospital, his family said Thursday.
She said he wishes to thank all the doctors and nurses, as well as the fire department and paramedics, for taking care of him during what he called a "health scare." They have not disclosed his symptoms.
Daley, who turned 80 this year, was elected mayor in 1989 and served for 22 years. In 2014 he suffered stroke-like symptoms.
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Northwestern Memorial Hospital is painfully familiar to the longtime mayor. It was there, in the cancer ward named for his late wife Maggie, that he announced in 2010 that he wouldn't seek a historic seventh term as Chicago's mayor.
His father, Richard J. Daley, died of a heart attack at 74 in 1976.
The Sun-Times Media Report contributed to this post.