Mr Limbaugh is one of the most popular radio hosts in the US
Right-wing US talk show host Rush Limbaugh is undergoing medical tests after being admitted to hospital in Hawaii with chest pains.
The Queen's Medical Center, in Honolulu, said Mr Limbaugh was "comfortable" and in "good spirits".
Hawaiian TV said Mr Limbaugh was taken from his hotel on Wednesday, where he was on holiday, in a serious condition.
Mr Limbaugh, 58, is highly influential in the US and his radio show is one of the most popular in the country.
It is broadcast on some 600 stations across the country and heard by more than 14 million people every week.
"He is being treated today, continues to rest comfortably, and remains in good spirits," the Queen's Medical Center said in a statement.
"Mr Limbaugh appreciates the prayers and well wishes of his many fans and supporters," it added.
Guest host Walter E Williams, standing in on Mr Limbaugh's radio show, described the radio star's pains as similar to the feeling of a "heart attack coming on" but said doctors hadn't confirmed whether the presenter had suffered a heart attack.
"He's going to take a lot of tests," Mr Williams told listeners on Thursday.
"Rush will have a complete examination today, and we'll know more and will keep everyone informed."
Honolulu TV station KITV reported that paramedics had taken Mr Limbaugh to hospital in "serious condition" from the Kahala Hotel and Resort.
Mr Limbaugh's spokesman, Kit Carson, said it was unclear if Mr Limbaugh would be able to return to work as scheduled on 4 January.
Mr Limbaugh, who is seen by some as as the unofficial voice of the US Republican party, has been a vocal critic of US President Barack Obama and is known for his fiery lambasting of Democratic politicians and policies.