Linda Ronstadt, rock 'n' roll's perennial sweetheart, wants to be an opera star, too.

Along with other rock stars, her record sales have slowed, and she can no longer fill the big arenas as a matter of course.

But rather than simply recycling and refining her successful pop formulas, Ronstadt has gone back to school, learning to sing all over again. And while Mick Jagger is still her hero -- and her friend -- she has found new inspiration in the life and work of legendary opera singer Rosa Ponselle.

Linda Ronstadt, who looks at you with eyes that don't seem to tire in directness, is resting after a six-night run at Radio City Music Hall. Tonight she is to be in Washington for a one-night Capital Centre benefit for the Ronald McDonald House.

Her hotel room is in disarray, a work home away from the private apartment she keeps on the edge of Central Park. Somewhere under the open suitcases, breakfast remains, rented videotapes, cracked books and emptied purses is the key to Ronstadt's safety deposit box. Not a thing to lose, but a thing to rejoice in finding, which she does. The most popular female rock singer of the late '70s settles back and packs up a night's worth of videotapes: "Carrie," "Ragtime" ("a friend of mine did the soundtrack, it's great") and two Shirley Temple movies.

"I got hooked on Shirley Temple when I was sick two years ago," Ronstadt says, flicking back a loose strand of hair. "She was good -- I'm not kidding. She learned all those lines, she could sing and dance; and she sang really in tune, a little 3-year-old singing great. And she could really tap. I know a little about tap dancing . . . I know a whole lot about singing . . . and I know a whole lot about doing movies where you lip-sync. But she had to lip-sync and tap in sync, too, because all the tapping was prerecorded. It was astonishing, that she was able to do it and still look pretty natural."

Ronstadt's familiarity with singing goes back -- professionally -- to 1964 and the Stone Ponies; the tapping is a mystery, but the lip-syncing was done for the upcoming movie version of Gilbert and Sullivan's "Pirates of Penzance," the show in which Ronstadt moved from pop to Papp (Joseph, that is) and astounded critics and fans alike with her facility for light opera. That move kept her out of the pop eye for most of a year. Her recent album, "Get Closer," was Ronstadt's first studio album since 1980, and although it recently turned gold, it might not extend her streak of platinum albums, a string that stretches back to 1976.

It looks for now as if Ronstadt will follow her lifelong habit of not listening to her own records by not watching her own film.

"It was so tedious," she says of the lip-synching process. "There isn't anything more tedious to do. You could make five regular movies for that effort." She knows she's not happy with her costumes; she's also not sure that she took advantage of instincts brought out during the six-month run of "Penzance."

"I'm positive I did it better in the theater," she says. "I'm just certain I did. I loved doing eight shows a week, enjoyed every single instant of it, every minute of rehearsals." When her selection as the Victorian ingenue was first announced, there had been audible guffaws in many segments of the theater world. But she won her critics over and ended up with renewed control and maturity.

Throughout her career, Ronstadt has done her share of second-guessing, mostly on a personal, rather than a professional, level. But those days seem solidly behind her now, and while she is far from abandoning her pop career, she is willing to take some artistic chances.

She will work with Papp again this summer, doing a more challenging repertory program of Bertolt Brecht-Kurt Weill works, "The Seven Deadly Sins" and "The Rise and Fall of the City of Mahagonny." The pay will probably match her $400 a week for "Penzance." ("I did it for love. It costs me money every time I talk to Papp.") She will finish an album of '30s and '40s pop standards with longtime Frank Sinatra arranger Nelson Riddle. And she may eventually tackle opera without the lightness. But more on that later.

Her current tour, she insists, is not a pop swan song. "There's room for everything," she insists, "and I've got plenty of time. I don't think my audience is going to go away because I didn't put out an album for a year and a half. It just can't be the same thing all the time. It can, of course, but I've always loved that other kind of music, and I've always sung it in one form or another."

The difference, Ronstadt feels, is that now she's beginning to sing it well. For "Penzance" she took the first voice lessons of her career, "a real revelation, just about the most important thing that's ever happened to me. I'm glad I never studied before, because it would have made it harder to evolve whatever it is about my style that makes it unique. Seeing a voice teacher is about as hard a seeing a shrink," she laughs. "And it's a very hard thing to teach, because while there is a real, tangible physical apparatus that controls what you do, you can't really see it or feel it or touch it."

Her teacher is Marjorie Rivingston, who has worked with singers as disparate as Beverly Sills and Bette Midler. "She's articulate, with a teacher's gift for using either positive reinforcement or being brutally frank without being destructive. I see all this as a remake of '42nd Street,' where a girl comes to Broadway and desperately needs help, a teacher to show her what to do. I've learned how to do stuff in rock 'n' roll, how to hit notes without making a legit sound but using some tricks. I've learned techniques from within so that I have way more range than before without having to scream as hard. It's just so interesting. Every time I go in there, I learn how to improve a tiny bit."

Somewhere down the line, when confidence is joined by mastery, Ronstadt hints that she wants to do real opera. "I don't like light opera, I wish I did. 'Penzance' was darling and fun, but that stuff isn't written for a singer in the way a Puccini opera -- where the vocal facility is written into the music -- would be. It's written purely for entertainment. I'd rather sing a Puccini aria any day. I'm very interested in opera, but not the way it's been performed over the last 60 or 70 years. I want to make it accessible, less ponderous. I'm going to try . . ." Her voice trails off as if too much has been said. Might it be Puccini? "Well, he is most accessible to me and the public . . ."

Ronstadt still confesses to nightly stage fright, but she rolls her eyes at the mention of Carly Simon's similar phobia; it's Rosa Ponselle whom she cites as a model. Like Ponselle, "I conquer it every night by going out on stage." Ponselle fought the situation for 30 years; Ronstadt's only been at it for 18. She seems determined to proceed as a singer -- not as a star, a role she has never been comfortable with. Her risk-taking on Broadway and in the studio enhance this approach.

She's not interested in more movies, though early reports indicate she's excellent in "Penzance." Movies represent more of the "static" art she's trying to avoid; it's why she can't listen to her own records. "It's not natural," she insists. "When anybody sings, it's alive, different every time. To have it frozen where it will never change is like watching a sunset instead of a photo of a sunset that never changes."

She is, she insists, a singer from the start, a singer to the end. "When I was 2, I was a singer, not a star. I don't think I chose it, it chose me. When I was in the first grade, intimidated by the nuns, I couldn't even add. Couldn't walk up to the blackboard to do sums. But if they asked me to sing songs, I'd jump up and sing right out. You can't walk around starring all day long, but you can walk around singing. You don't star in the shower, but you sing."

Things change. Once a yearly cover story in Rolling Stone, she has missed for the first time since 1976. She used to be called the million-dollar singer, but nobody's million buys what it used to. Still, the pop career that ballooned in the '70s is not about to disappear (Ronstadt is working hard to make sure her natural pop tendencies don't get trained out). She's not worried that there have been empty seats on heavily advertised shows where once word-of-mouth could guarantee sellouts. She's not thrilled with arena rock, either as performer or fan -- "I stopped seeing a lot of my friends in concert when they stopped doing smaller venues," she says.

"It used to be a sense of the event," the 35-year-old Ronstadt says of her concerts. "But my fans have grown up; they're mailmen and dentists and housewives and bankers. They have children, they don't want the hassle of parking and baby sitters and arenas."

They've settled down, and Ronstadt insists she has, too. "I've been settled for the last 15 years in my career. People have a tendency to think of settling down as living in one place or with one person, but I'm like a sailor who settles down to live at sea."