The reporter, who is still waiting to hear what evidence is being used against him, has been in a Spanish prison since last year on charges he belonged to al-Qaida.
Alluni is best known for interviewing al-Qaida leader Usama bin Ladin shortly after the 11 September 2001 attacks and was first arrested in September 2003 at his home in Granada.
He was bailed around a month later because of a serious heart condition.
Spanish police rearrested him in November 2004 on charges that he provided money and information to al-Qaida as well as recruited fighters. Alluni, who holds Spanish nationality, says he is innocent. The Aljazeera journalist is being held in a small cell and is allowed out four hours a day on condition that he does not communicate with anyone.
"Aljazeera Channel has been monitoring with heightened alarm the health situation of its reporter Taysir Alluni who has been incarcerated for the past couple of months in a solitary cell in a prison compound on the outskirts of Madrid," Aljazeera said in a statement.
"Taysir's wife and his lawyers have approached the relevant authorities in Madrid with a request that he be examined by heart and back specialists. No response has been forthcoming, although Taysir's health has been deteriorating, and is threatening further serious complications," it said.
But a spokeswoman for the Spanish prison service said Alluni did not need hospital treatment.
"He has his heart problems but he does not need to be hospitalised. He has not had any serious incidents," she said.
A spokeswoman for the prison system confirmed in November that Alluni and 85 other suspects were being held in isolation.
Aljazeera urged Spanish authorities to consider Alluni's situation "in a humane and compassionate way" and called on international humanitarian and media organisations to intercede.
The Syrian-born father of five's initial arrest sparked outrage among Arab human rights groups, journalists and colleagues at Aljazeera who called it an attack on press freedom.