The former New York mayor left the hospital through its emergency entrance, driving off in a waiting black sedan, the St. Louis Post-Dispatch reported.

"I'm feeling fine, thanks to the hospital," Giuliani said. "They did a great job."

Giuliani was admitted to Barnes-Jewish Hospital about midnight Wednesday. He said he began feeling ill after a day of campaigning in Missouri and, after his flight took off for New York, it worsened, a campaign spokesman said.

The plane returned to St. Louis and Giuliani contacted his personal physician who told him to go to the hospital.

"After precautionary tests the doctors found nothing of concern at this time and Rudy will be going back to New York (Thursday)," his campaign spokeswoman Katie Levinson said. "He is in high spirits and is grateful to the doctors and nurses who checked him out."