Conductor Zubin Mehta will not consider assuming the post of musical director of the Los Angeles Philharmonic Orchestra after Andre Previn steps down in 1991. "I would go back to guest conduct, but it's completely inconceivable that I'll go back as musical director," he said in an interview in Tel Aviv, where he was rehearsing the Israel Philharmonic Orchestra, which he serves as musical director. Mehta said that when he leaves the New York Philharmonic (where he is also music director) in 1991, "I will retire as a music director in North America . . . . I will have closed a certain chapter in my life." He intends to maintain his 28-year association with the Israel Philharmonic and dedicate the rest of his time to guest conducting, opera and other projects. Of Los Angeles, he said: "Two years ago when I conducted the Los Angeles Philharmonic for six weeks, I was in a state of euphoria. They have remained one of the world's great orchestras." Mehta will lead the Israel Philharmonic on a North American tour that includes mid-March concerts in Los Angeles, Costa Mesa, San Diego and Palm Springs.