Arians had been the Colts interim coach for 12 games while coach Chuck Pagano was battling leukemia. He turned the team back over to Pagano last week.
The Colts play the Ravens at 1 p.m. today in M&T Bank Stadium.
A Colts representative said Arians was admitted to the hospital this morning and will be held overnight for observation. The Colts said he is doing well. No other information was available.