The inevitable clash between the two conductors came quickly, and it was over
Tristan and Isolde. As part of his contract, Toscanini had insisted that he be given a new production of
Tristan; Andreas Dippel was given the job of breaking the news to Mahler. As could have been expected, Mahler was furious and refused to agree. He considered
Tristan, which had been his debut at the Met, to be his territory, and he threatened to resign.
For the 1908-9 season, Mahler prevailed, but in 1909, Toscanini conducted
Tristan. Suffering renewed symptoms of his heart problem, Mahler did not have the strength to fight another battle. Later, he accepted an invitation to become music director of the newly formed Philharmonic Society of New York, while Toscanini settled in at the Met.
http://hornworld.wordpress.com/2010/09/21/mahler-and-toscanini-at-the-met/