Former prime minister Manmohan Singh was admitted to the AIIMS here on Sunday evening after he complained of uneasiness, sources said.
Singh, 87, is under observation at the cardio-thoracic ward of the premier hospital, they said.
"All his parameters are fine. He is under observation at the AIIMS," a source close to him said, adding that he was taken to the hospital after he felt "uneasiness".
He was admitted under Dr Nitish Naik, a professor of cardiology at the All India Institute of Medical Sciences (AIIMS), around 8.45 pm.
Singh is a senior leader of the opposition Congress and is currently a Member of Parliament in the Rajya Sabha from Rajasthan. He was the prime minister between 2004 and 2014.
In 2009, Singh underwent a successful coronary bypass surgery at AIIMS.