"It's a fabricated story - only rumors," Galeb Abu Zainab, a member of Hezbollah political office, was cited as saying by the Lebanese An-Nahar newspaper.
The Iraqi website Almalaf reported Wednesday that Hassan Nasrallah had been poisoned last week and that Iranian doctors had rushed to Lebanon in an attempt to save him.
It cited undisclosed sources as saying Israel was responsible for the assassination attempt.
"The only party that can benefit from these rumors is the Israeli intelligence service," Zainab said.
Nasrallah lives in hiding in Beirut and rarely appears in public because of the fear of Israeli assassination attempts.
His predecessor, Abbas al Musawi, was killed in an Israeli airstrike in southern Lebanon in 1992. Nasrallah's deputy, Imad Mughniyeh, was assassinated in February in Damascus.
Hezbollah has vowed revenge.
