MAYERLING, Austria, March 8— One hundred years ago, the heir to the Austro-Hungarian throne brought a 17-year-old mistress to his hunting estate in this sylvan village, shot her through the head and turned the gun on himself. Or did he? Was Prince Rudolf murdered, as die-hard monarchists hold? Or was he really killed in a drunken brawl with the uncles of his mistress, young Mary Vetsera? Or did he shoot himself when Mary bled to death after a secret abortion? Was he deranged, as his father declared? A drug addict? Syphilitic? With the 100th anniversary of the deaths at Mayerling, one of Austria's most durable mysteries has been revived with the sort of fascination the Austrians lavish on the juiciest of scandals. A stack of new books and a television program have reopened all the old theories and added a few brand-new ones. Convent Where Lodge Stood
''Before, there were 20 versions and now there are 60,'' lamented Martha Rossler, a retired woman who acts as guide for the steady trickle of visitors to the Carmelite convent established by Emperor Franz Josef on the site of the rustic manor, 20 miles south of Vienna, where his son and heir killed and died.
The altar of the convent church stands over the spot where the bodies were found, in what was once the 31-year-old Prince's bedroom. Visitors proceed to adjacent rooms to examine a few items of furniture and china from the old hunting lodge and a few prints.
The display is humble. But for a small country steeped in the grandeur of a vanished empire whose magnificent vestiges and myths still dominate the land, Mayerling is a memorial to an extraordinary tragedy and an extraordinary scandal.
In the universities it is taught as the first clear symptom of the disintegration of the Hapsburg empire. Hungarians in the Act, Too
The 100th anniversary was even accompanied by a dollop of glasnost. Hungary, whose Communist leaders have suddenly shown a revived interest in their imperial legacy, co-produced a major television documentary on Prince Rudolf.
So intense was the renewed wave of curiosity that the church and the Hapsburg family had to intercede to prevent Mary Vetsera from being exhumed in nearby Heiligenkreuz to check if she was, as the prevalent version goes, shot through the left temple.
The anniversary was also the occasion for an international conference of experts on suicide at Mayerling. It was attended by historians, psychiatrists and others, who concluded that Prince Rudolf did kill the girl and himself in a suicide pact. A Sampling of Theories
The century of speculation has been sustained by Emperor Franz Josef's singularly unsuccessful attempt to still the Europe-wide furor by suppressing the findings of an inquiry he ordered, as well as the letters Prince Rudolf wrote before he died - including, the story goes, farewell letters.
Of the new hypotheses that have blossomed with the anniversary, the most inventive has to be that of Clemens M. Gruber, an opera archivist in Vienna, who postulated in his book ''The Fateful Days of Mayerling'' that the Prince was shot by relatives of Mary Vetsera who broke into their room after a drinking party.
Then there is Gerd Holler, a physician who argues in ''Mayerling: New Documents on the Tragedy 100 Years Later'' that Rudolf had arranged an abortion for Mary, and committed suicide after she bled to death.
The version generally considered most authoritative is that of Brigitte Hamann, a historian who described the heir in her book, ''Rudolf, Crown Prince and Rebel,'' as a progressive thinker thrown into despair by the policies of his autocratic father and the conservative court, as well as by the syphilis.
''Oh no, you're not going to make me talk about Mayerling again,'' Mrs. Hamann exclaimed to an inquiring reporter. ''There really is no new information, only the old things retold again and again. The fact is he killed the girl first and then shot himself.'' What the Pope Was Told
The Emperor never denied the suicide, she said. But to insure his son a Roman Catholic funeral, he had to assure the Pope that the Prince did not kill the girl, and that he shot himself ''in a deranged state of mind.''
The Vatican accepted the version, and Rudolf now lies with 137 other Hapsburgs in the imperial vault in the crypt of the Church of the Capuchins in Vienna.
According to Mrs. Hamann's research, Prince Rudolf had in fact contemplated suicide for at least a half year before his death. He initially asked the first love of his life, one Mitzi Kaspar, to share his fate, but the 24-year-old woman declined.
By contrast, 17-year-old Mary Vetsera, the dark and pudgy daughter of an Italian-Armenian baron, was madly in love and willing to do anything for Rudolf. So on Jan. 29, 1889, he took her to his favorite hunting lodge, and the next morning shot her and himself in their bedroom. Alienated From His Father
Exactly why the Prince wanted to die may never be fully understood. The experts who gathered for a symposium on the anniversary at Mayerling concluded that Rudolf suffered from a profound sense of political isolation.
An educated man of liberal views, he was totally at odds with his father and the court.
His search for a woman to die with, the experts thought, apparently resulted from a fear of dying alone. But Mary Vetsera, for whom Rudolf was virtually an idol, was not the woman he loved.
It was to Mitzi Kaspar that he addressed a passionate love letter on the night before he killed himself. The letter has disappeared, as have other documents from the suicide, but people who read it said it demonstrated both a passionate love and a clear state of mind.
Photos of Prince Rudolf of Austria-Hungary; his mistress, Mary Vetsera; the hunting lodge where Prince Rudolf supposedly killed Mary Vetsera