Trinamool Congress chief Mamata Banerjee, who was admitted to a city nursing home early on Thursday after she complained of chest pain and breathlessness, was stable, doctors said.
In a medical bulletin, doctors said her condition was improving and there was no internal bleeding.
They have advised Banerjee a few days' rest.
She was admitted to the Belle Vue Nursing Home in central Kolkata.
Banerjee complained of the problems late Wednesday night after attending a rally against the Singur land grab with Congress leaders during the day where she looked visibly indisposed.
The MP was allegedly manhandled during her sit-in to resist the transfer of fertile farmlands to Tata Motors for a car factory.
Her family and party colleagues attributed the illness to the assault on her at Singur.