This article, which includes publications of hitherto unknown documents from the archives of St. Petersburg and Vienna, describes a characteristic episode from the activities of Gustav Mahler as the head of the Court opera in Vienna. It has to do with the preparation in the fall of 1899 of the Viennese premiere of the opera The Demon by Anton Rubinstein, in connection with which Mahler was require to study the score thoroughly. His letter to the Direction of the Imperial Theaters in St. Petersburg sent to the director of the Russian Opera Osip (Joseph) Palechek addresses the problem of the realization of one incoherent fragment (the «tam-tam» on the stage, utilized in the beginning of Act 3). The cited documents shed light on the practice of production that was current at the end of the 19th century. For the elucidation of the essence of the question posed by Mahler information is cited about the history of the text to Rubinstein’s opera.