Julius Rudel is leading his New York City Opera Company back into the Kennedy Center again this week. You can say "his" because this is Rudel's 20th year in charge of the famous New York company, and he says there is no truth whatsoever to the rumor that he is considering leaving. You say "back again" because the annual visits of the company, whose home base is Lincoln Center's State Theater, have become popular attractions at the Kennedy Center.

During these past 20 years, however, Rudel has not been spending all of his time with the New York City Opera. He has been very busy helping to direct a number of important American musical activities.For various periods, he was musi director of the Cincinnati May Festival, the Caramoor Festival, artistic adviser to Wolf Trap Farm Park (a title he still holds) and artistic adminstrator to the Kennedy Center. He has served on the board of directors of the National Opera Institute since its inception over seven years ago and is still its board chairman.

With increasing frequency in recent years, he has conducted opera in Paris, Salzburg and Vienna and in recording studios. From the latter have come historic releases of Handel's "Julius Caesar," Massenet's "Manon" and Boito's "Mefistofele." This last major item is one of the operas Rudel will conduct here this week with his company. During the coming summer a new recording of Charpentier's "Louise" will be recorded under Rudel's baton with Beverly Sills and Nicolai Gedda as the young lovers. In planning the company's current successful revival of "Louise," Rudel decided to stage it himself as well as conduct it, for its Parisian atmosphere makes it among his favorite works.

If you ask Julius Redel how he ever got into "all this," a rather special kind of smile comes over his face and he begins to twll you about the La Puma Opera Company. This was a company in New York City right after World War II when Redil, who had come to this country from his native Austria, had finished his studies at the Mannes School of Music. Naturally he was looking around for work. The La Puma Company was one of a number of similar outfits that you could call enterprising, struggling, living from hand-to-mouth, the kind of company that used to let people sing with them who paid for the privilege.

Rudel was asked if he would like to conduct for them, and of course he said he would. But he had some reservation about doing it under his own name. So - in one of ther great traditions of opera that led John McCormack to make his debut under the name of Giovanni Foli and a tenor named Joseph Benton from Oklahoma to make his under the label of Giuseppe Bentoneli and a soprano named Marion Clark to become Franca Somigli - Julius Rudel simply turned his name around, gave it an Italian twist and conducted as: Rodolfo di Giulio.

It did not take long, however, for a musician born in Vienna, with opera running in his veins, to attach himself to the youthful New York City Opera that was frequently winning better notices, as Rudolf Bing used to complain, that the Metropolitian.

Early in hs direction, Rudel's flair for unusual repertoir and shrewd casting became one of the things that attracted subscribers to the New York City Opera who loved "Carmen" and "La Boheme" but wanted to hear some operas they had not heard before. For them and thousands of others, Rudell scored a smash hit when he paired Sills and Norman Treigle in Handel's "Julius Caesar." But he could also make a standard item out of the unknown opera "The Makropoulos Case," thanks to the singular talents of Marilyn Niska who found in Janacek's score one of her most congenial roles.

Only two weeks ago Rudel's company gave the world premiere - no unusual event for them - of "Lily," by Leon Kirchner, with the composer conducting the music he wrote out of Saul Bellow's novel, Henderson, The Rain Maker." But Rudel never reaches one world premiere without having another in mind. Alberto Ginastera, composer of three successful operas, "Don Rodrigo," 'Bomarzo" and 'Beatrix Cenci," is at work on a "Barabbas" for the New York company. Yet with all his enthusiasm for new and unfamiliar operas, Rudel can astonish you in the next sentence by saying that he is going to conduct Flotow's "Martha" in Vienna next year because the Viennese have decided to make it a part, if an unfathomable one, of their Schubert festival. "And besides," Rudel adds with that smile again, "any time you are asked to conduct anything in the Theater an der Wien . . .!"

While he is in Vienna, Rudel wil also conduct Schubert's Mass in A Flat for an ecumenical celebration of Mass in the Stadthalle. That kind of assignment is only another reminder of the many nonoperatic activities that regularly occupy Rudel. At Wolf Trap he has led a Mahler cycle that will this summer include the Fourth Symphony and the song cycle, "Des Knaben Wunderhorn."

With great relish he says, "I have suggested to them at Wolf Trap that when the Mahler cycle is over I should follow it with a cycle of Dvorak - there is such a close feeling between mahler and Dvorak, and we are going to do the New World Symphony this year, so with this parallel, why not a cycle, and include not only all of the nine symphonies, but the large choral works also, and one of the operas - 'Dalibar.'" That is the name of an opera that the late great conductor Bruno Walter used to acclaim as one of the supreme operas of the 19th century. We can hope Rudel gets his Dvorak cycle at Wolf Trap.

Like everyone connected with the National Opera Institute, Rudel is enthusiastic about much that it has accomplished, especially in tha area of helping young operatic artists. And while he is no longer officially attached to the Kennedy Center as he was even several years before it opened, his eyes light up at any mention of what it is helping to bring about, and at the thought of conducting in it, as he will be this week.