The Kremlin said the 66-year old president's illness was not serious but Mr Yeltsin had to cancel plans to record a radio broadcast to mark today's Constitution Day public holiday.
The official television footage, released without sound, showed Mr Yeltsin greeting his chief-of-staff, Valentin Yumashev, at the Barvikha sanatorium outside Moscow, where the president is expected to stay for up to 12 days.
Mr Yeltsin moved relatively freely but looked grave and did not smile.
"The president is in some discomfort and has a temperature of 37.3C," said a Kremlin statement.
Bulgarian President Petar Stoyanov, who had been due to visit Moscow next week, told reporters in Sofia he had spoken for 20 minutes by telephone with Mr Yeltsin yesterday and that his trip had been postponed until February or March.
The Kremlin dismissed a Western media report saying Mr Yeltsin had suffered a renewed bout of heart trouble.
- Reuters, MoscowReuse content