A British magistrate will decide Friday whether Augusto Pinochet can be extradited to Madrid, where a Spanish magistrate wants him tried for crimes allegedly committed during his 17-year rule in Chile. But even if the decision goes against the former dictator, Chilean authorities are trying to build a case for his release based on humanitarian concerns over his failing health.

In its attempt to win Pinochet's release, the Chilean government has cited mainly national sovereignty grounds and has pledged to put him on trial here, but thus far without success. So it has taken a new tack, stressing that Pinochet's health--he is 83--has deteriorated alarmingly and that his death abroad may have the unwanted effect of turning him into a martyr back home, according to Chilean Foreign Minister Juan Gabriel Valdes.

If the British magistrate rules in favor of Pinochet, Chilean authorities hope the new initiative will allow them to circumvent a promised appeal by the Spanish magistrate, Baltasar Garzon, that could keep Pinochet in London. And if the ruling goes against Pinochet, the new strategy may be his best chance to avoid months of legal appeals--and perhaps the trial in Madrid that Garzon is seeking.

"Nothing else the Chilean government has done to free Pinochet has worked," said Ricardo Israel, director of the University of Chile's Political Science Institute. "So it has come down to requesting mercy. . . . Everyone sees this as the last best effort to get him freed."

Pinochet, the most notorious of South America's military dictators of the 1970s and '80s, was arrested while visiting Britain almost a year ago, after Garzon requested Pinochet's extradition to try him in Spain for some of an estimated 3,000 killings and missing-persons cases blamed on his government. Since then, although his age and health have often been cited in legal arguments for his release, they have never been the focus of the Chilean government or Pinochet's lawyers.

Now that is changing. Valdes said in an interview that the stress of legal proceedings and detention under house arrest have made Pinochet's medical condition "potentially fatal." His lawyers say he has blacked out twice--episodes they describe as "mini-strokes"--in the past two weeks and has lost feeling in one leg.

Pinochet has been examined by at least four physicians, including two British doctors described here as impartial, who have diagnosed his condition as grave enough to keep him from appearing in court Friday to hear the magistrate's ruling. Pinochet has diabetes, wears a pacemaker and is being treated with anti-depressant drugs, Valdes said.

Valdes said British Foreign Minister Robin Cook also has expressed concern for Pinochet's health in recent conversations but gave no indication that British officials--namely Home Secretary Jack Straw, who has jurisdiction over extradition matters--will intervene in Pinochet's case.

Still, Valdes expressed the hope that British opposition to a humanitarian release may be easing. "I think there is a better understanding of Pinochet's health risk . . . [and] neither the British nor we want him to die in London," Valdes said.

Valdes said Pinochet's death abroad would halt gains made here in investigating the cases of more than 1,200 dissidents still unaccounted for from Pinochet's dictatorship, which ended in 1990. Since Pinochet's detention, Chile's emboldened civilian judicial system has arrested or indicted 18 former military officers, including former generals, in connection with crimes committed during Pinochet's rule. The military command also has begun unprecedented negotiations with human rights lawyers investigating the whereabouts of missing persons, or the "disappeared," as they are called here.

"If Pinochet dies in London, we won't have the chance to deal with this issue ourselves, and that is what we as a nation must do," said Valdes, a onetime dissident and assistant to Orlando Letelier, a former Chilean ambassador to the United States who was assassinated in Washington in 1976 by Chilean agents.

Yet some are not so optimistic about British cooperation. Pinochet's youngest son, Marco Antonio Pinochet, told reporters this week that he believes his father will die in London as the legal battle continues. And British officials, along with Spanish authorities, have said publicly that they will not intervene in what they view as a judicial process.

But the Spanish government, which is deeply divided over Garzon's attempt to put Pinochet on trial, is facing pressure from business interests to release Pinochet. Spanish corporations own the power, water and phone companies in Chile and control several of the largest banks in Santiago, the Chilean capital. "Spanish executives here are applying pressure back home because they see this case as damaging their standing in Santiago's business and conservative political circles, where Pinochet is very much loved," said Enrique Correa, a Chilean political consultant.

The question of exactly how ill Pinochet is also remains a subject of debate. Although sources close to him say he is seriously ill, others maintain that he is merely suffering infirmities consistent with old age. They say that if Pinochet were freed in London to be put on trial here, the strain on his health would be the same in a Chilean court as a Spanish one.

"What we do know is that he is not on his death bed," said Chilean Congressman Juan Pablo Letelier, the son of Orlando Letelier. "He never showed respect for life, and the [notion] that he should be released now for humanitarian reasons is difficult to comprehend."