Foreign Minister Walid al-Moallem, who is in his 70s, is known for his hard-line stance against the opposition and is a close confidant of President Bashar Assad.
It is not clear if the overweight al-Moallem has had heart problems in the past.
Al-Moallem was brought to the American University of Beirut Medical Centre, according to Lebanese security officials, speaking on condition of anonymity in line with regulations.
Lebanese lawmaker Assem Qanso, who visited al-Moallem today, told reporters outside the hospital that Syria's top diplomat is "in stable condition" and would undergo an operation.
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The security officials said that when al-Moallem arrived at the hospital, he was walking slowly. Officials at the AUBMC which has treated several ranking officials from Assad's government in the past refused to confirm that al-Moallem was being treated there, saying they give no such information to "third parties" but only to medial teams, the patient and family members.
Lebanese police and plainclothes officers were deployed outside the hospital in Beirut. Syria's ambassador to Lebanon, Ali Abdul-Karim Ali, was seen entering the facility earlier this morning, presumably to visit al-Moallem but did not speak to reporters.
A career diplomat, he served as ambassador to Washington for nine years, starting in 1990 during Syria's on-and-off peace talks with Israel.