Former Prime Minister Atal Bihari Vajpayee, 83, was admitted to the All India Institute of Medical Sciences (AIIMS)in New Delhi on Tuesday after complaining of giddiness.
Doctors said the Bharatiya Janata Party (BJP) leader's condition was stable.
"Vajpayeeji complained of giddiness and was feeling drowsy. He was admitted to our institute in the afternoon. He has been referred to the cardio thoracic and neuroscience centre," said Shakti Gupta, professor in charge of media.
"We conducted a few tests. We will also carry out a more thorough body scan. A team of cardiologists, neurologists and physicians are closely monitoring his condition," Gupta, who is also the chief medical officer of RP Centre of Ophthalmology, told IANS.
"The former prime minister is conscious, stable and all his vital signs are normal," he added.
The veteran political leader will be in hospital for at least one day, authorities said.