Lester Piggott is expected to remain in hospital in London for a "few more days" after being admitted over the weekend.
One of the sport's all-time great figures, Piggott, 83, was reported to be "in good shape" by his daughter Maureen Haggas, wife of Classic-winning trainer William Haggas.
Haggas told the Racing Post her father would be having routine tests, adding: "He'll be in for a few more days, but was taken in as a precautionary measure. He's in good shape considering he's been abusing his body since he was 16."
Piggott - who had a spell in intensive care in 2007 in a Swiss hospital with a heart complaint - won the Derby a remarkable nine times.
He was associated with many equine greats, including Nijinsky, dual Prix de l'Arc de Triomphe winner Alleged, triple Ascot Gold Cup winner Sagaro and the globetrotting wonder mare Dahlia.