Argentine President Cristina Fernandez de Kirchner is being treated for an "infectious fever" in a hospital in the capital, Buenos Aires, officials say.
Ms Fernandez, 61, checked herself into the clinic after feeling ill Sunday afternoon, an official statement explained.
The president has had a series of health problems, some of which have forced her to cancel official trips.
Last year she had an operation to treat bleeding on the brain.
She also missed this year's independence day celebrations due to a throat infection.
President Fernandez, of the left-wing Front for Victory party, was first elected in 2007 and then returned to power by a comfortable majority in 2011.
Her late husband, former President Nestor Kirchner, died after a heart attack in 2010.
On Monday, Cabinet Chief Jorge Capitanich said that he had spoken to her before and after she had gone to hospital but did not give any more details of her condition.