"President Carter is in very good spirits, appreciates all the good wishes being sent his way, and looks forward to getting back to his busy schedule," hospital officials said in a statement released Wednesday by MetroHealth Medical Center.
Hospital spokeswoman Susan Christopher declined to say whether Carter's condition had worsened since he was admitted for stomach problems on Tuesday morning. Carter became ill during a Delta Air Lines flight from Atlanta to Cleveland, causing rescue crews to rush him to the hospital after the plane landed.
Carter's medical team recommended that he stay a second night for additional monitoring, Christopher said. Earlier Wednesday, hospital officials said Carter was feeling normal and continued to be observed by doctors.
On Tuesday, Carter's grandson, Georgia state Sen. Jason Carter, said his grandfather was doing fine.
"He's definitely resting comfortably and expected to continue his book tour this week," Jason Carter said. "I haven't talked to him, but nobody in the family is concerned."
Mary Atkins, who had taken her daughter to the hospital for medical treatment on Tuesday, saw Carter from a nearby room and said he was up, walking around and saying he was ready to leave.
The hospital stay has interrupted Carter's tour to promote his new book, "White House Diary."
Planned book-signings in Ohio and North Carolina were called off Tuesday. His publisher also canceled scheduled events in Washington on Wednesday, including one at the Smithsonian Institution. The status of an event planned at a Columbia, S.C., bookstore Thursday was unclear.
Carter, a former peanut farmer elected to the White House in 1976, has spent his recent years pursuing peace and human rights, efforts that won him the Nobel Peace Prize in 2002.
In the new book, Carter said he pursued an overly aggressive agenda as president that may have confused voters and alienated lawmakers. But he said the tipping points that cost him the 1980 election were the Iran hostage crisis and the Democratic primary challenge by U.S. Sen. Ted Kennedy.
He is the author of more than 20 books, including the 2006 "Palestine Peace Not Apartheid," about his experiences in the Middle East, and the 2005 "Our Endangered Values: America's Moral Crisis."
Associated Press Writer Meghan Barr contributed to this report.