Parliament president and House speaker Chuan Leekpai was rushed to hospital on Tuesday evening with fever and chills, Thai media reported.
Mr Chuan, 83, was admitted to Ramathibodi Hospital in Ratchathewi district, Bangkok.
His personal physician said Mr Chuan will undergo a thorough examination, but his condition was not worrying.
Mr Chuan had suffered similar symptoms many times before, and he was told to visit the hospital immediately if he experienced them again, the physician said.
On Tuesday morning, the Democrat Party chief adviser joined a merit-making ceremony to mark the 90th anniversary of the parliament and performed his duties until evening.