Mr. Bush called Roberts Tuesday morning.
"The chief justice assured him that he was doing fine," White House press secretary Tony Snow said. "The president was reassured."
Roberts "sounded like he was in great spirits," Snow said, relaying details of the phone call.
Doctors who examined Roberts called the episode a "benign idiopathic seizure," meaning they found no tumor, stroke or any other explanation.
Roberts, 52, had a similar, unexplained attack in 1993.
The seizure Monday caused the chief justice to fall on a dock, where he sustained minor scrapes, Supreme Court spokeswoman Kathy Arberg said.
Once at the hospital, he underwent a "thorough neurological evaluation, which revealed no cause for concern," Arberg said.
She said he was kept overnight at the Penobscot Bay Medical Center in Rockport for observation.
"Idiopathic means they really don't know what caused it," Dr. Emily Senay told CBS' The Early Show, and added that the seizures could have a genetic component to them.
By definition, someone who has had more than one seizure without any other cause is determined to have epilepsy, said Dr. Marc Schlosberg, a Washington Hospital Center neurologist who is not involved in the Roberts case.
Whether Roberts will need anti-seizure medications to prevent another is something he and his doctor will have to decide. But after two seizures, the likelihood of another at some point is greater than 60 percent.
Epilepsy is merely a term for a seizure disorder, but it is a loaded term because it makes people think of lots of seizures, cautioned Dr. Edward Mkrdichian, a neurosurgeon at the Chicago Institute of Neurosurgery and Neuroresearch.
Still, Mkrdichian said anyone who has had two otherwise unexplained seizures is at high risk for a third, and that he puts such patients on anti-seizure medications.
"Having two seizures so many years apart without any known culprit is going to be very difficult to figure out," agreed Dr. Max Lee of the Milwaukee Neurological Institute.
The incident occurred around 2 p.m. on a dock near Roberts' summer home in Port Clyde on Maine's Hupper Island. He had just gotten off a boat and was returning home after running errands, Arberg said. Port Clyde, which is part of the town of St. George, is about 90 miles by car northeast of Portland, midway up the coast of Maine.
Roberts was taken by private boat to the mainland and then transferred to an ambulance, St. George Fire Chief Tim Polky said.
"He was conscious and alert when they put him in the rescue (vehicle)," Polky said.
Named to the court by President Bush in 2005, Roberts is the youngest justice on a court in which the senior member, John Paul Stevens, is 87. Mr. Bush was informed of the hospitalization by his chief of staff, Josh Bolten, the White House said.
Roberts is the father of two young children.
CBS News chief White House correspondent Jim Axelrod reported that the 1993 episode, which occurred while Roberts was playing golf, did not come up during his confirmation hearings because it was thought to be an isolated event.
Larry Robbins, a Washington attorney who worked with Roberts at the Justice Department in 1993, said he drove Roberts to work for several months after Roberts' seizure that year. Robbins said Roberts never mentioned what the problem was and he never heard of it happening again.
In 2001, Roberts described his health as "excellent," according to Senate Judiciary Committee records.
Roberts became chief justice after the death of William Rehnquist in September 2005, although Mr. Bush had first chosen him to take Sandra Day O'Connor's seat when she announced her retirement earlier that year.
Roberts has led the Supreme Court to a more conservative stance. Helped by Justice Samuel Alito, who won confirmation in early 2006, conservatives have won twice as often as they lost on the Roberts-led court. The 2006-07 term brought limits on abortion rights, restrictions on school integration programs and greater freedom for political advertising.
Roberts earlier served as an appellate judge in Washington and spent more than a decade before that as a lawyer at the Hogan and Hartson law firm, where he specialized in arguing cases before the Supreme Court.
Roberts also served in the Reagan and Bush administrations in the 1980s and 1990s. He was a clerk for Rehnquist after graduating from Harvard Law School.
Roberts spent a couple of weeks in Europe in July, teaching a course in Vienna and attending a conference in Paris. He was at the court in Washington late last week.